What does it mean for consumers that Apple has released MacOS kernel as open source?
Clarification:
Apple has always released the kernel of MacOS in an open source form.  The kernel is named Darwin. Darwin has a target of x86.  It can be ported to other processors, but the open source variant of Darwin that Apple controls and releases was only available for x86.
However, iOS, which uses ARM processors, has used Darwin since its inception.  Apple has never released the ARM port of Darwin, even though they obviously have one since they're using it in millions of iOS devices already in consumer hands.
This announcement is that Apple is now releasing the ARM pieces of Darwin that they previously withheld.
Darwin is just a kernel, it doesn't include most of what makes the MacOS or iOS operate as a consumer expects them to.  A kernel is not a complete operating system.
Further, Darwin is not a complete kernel.  Apple doesn't release many portions of it.

What does it mean for consumers that Apple has released MacOS kernel as open source?

Not much, initially.  It will not affect the availability of hardware or software, upgrades, and probably won't even have a substantial impact on jailbreaking or similar alternate uses of apple devices.

Does this mean we'll see other Computers (which are not Apple built) run the MacOS?

It does not affect the availability of non Apple hardware devices running Apple software or operating systems.  What's already available won't become more or less available or more or less legal.

If not, then what would happen?

The most likely explanation, and how consumers will benefit, comes from an earlier release of code Apple usually holds close to its chest, &quot;Apple confirms iOS kernel code left unencrypted intentionally&quot; which suggests:

Apple has begun to shift towards greater transparency, particularly on security issues, in the wake of its battle with the FBI over unlocking an iPhone used by the San Bernardino shooter. When the FBI attempted to compel Apple to unlock the phone, CEO Tim Cook penned a rare open letter to Apple’s customers, explaining his decision to resist. “We feel we must speak up in the face of what we see as an overreach by the U.S. government,” Cook wrote. (The FBI eventually dropped its request after paying a third party to break into the device.)
Opening up the kernel’s code for inspection could weaken the market for security flaws like the one the FBI is presumed to have used to get into the San Bernardino iPhone. If flaws are revealed quickly and widely, it will reduce the prices law enforcement and black markets will pay for them — and it could mean quicker fixes for Apple’s customers.

So the consumer benefit is that more eyes, specifically security researchers, will be on the code and hopefully this will bring to light weaknesses in security, allowing Apple the ability to improve security for customers rather than finding out their security is broken when a government or third party is able to take away someone's rights without due process.
Whether this plays out positively, though, depends on researchers volunteering to comb through the code.

From what I understand, Unix kernel is released too, and Linux was created. Should the same happen to Apple's kernel?

Apple started with the original BSD code, which, using the BSD license, doesn't require that companies or individuals release their changes to the code to the people they distribute compiled software to.  Indeed, they chose this over the Linux kernel specifically so they could keep certain intellectual property secret (among many other reasons, of course).
So they do not have a legal obligation to release all their code. Some of the software they use does have other licenses, such as GPL, which does require they keep it open source. Much of these are available from Apple now.
This does, however, also point to another possibility: many kernel pieces are released so developers can develop MacOS hardware and specialized software.  This is much deeper into the system than app developers are allowed access to for iOS devices.
This release, then, suggests one or both of two strong possibilities (and this is complete conjecture):

Apple is preparing to release MacOS computers using their A series ARM processors.
Apple is preparing to allow developers to make deeper-level software for iOS devices.

I'd say MacOS computers using ARM processors is more likely, as the latest A11 chip, used in the iPhone 8 and X, beats the latest Intel i5 chip, used in the 2017 Macbook 13&quot;.  Apple still has a ways to go before they can replace Intel processors along their whole line, however we may see, within a year or two, low end MacOS computers with A12 or A13 chips, replacing the Intel chips previously used.
This would provide significant benefit to Apple (particularly its bottom line).
However, without an official statement there's no way to know what reasons Apple has for releasing the ARM portions of its already largely open source Darwin kernel.
The consumer will be impacted very little, if at all, and if so only indirectly.