what does "ports can be registered" mean? For circuit design
A registered output port is one where the data lines are driven according to the contents of a register that can be written and read just like a memory location.

The simplest form of output port is just a latch that can only be written to. In an embedded system, the individual port lines are often used for different functions and may well be controlled by different parts of the code that know nothing about each other, so it's important that you can change the state of individual lines without disturbing the others.

If you have a write-only output port you would need to use a memory location to remember the state of all the output pins so you can read the memory, change the bit(s) you want and then write the new value to the port and back to the memory location for next time.

A registered port lets you read the current outputs directly from the port, change the bit(s) you want and write them back again. Many processors have bit set and reset instructions to do this quickly and efficiently.