What prevents water from getting underneath these roof shingles?
Yes, there are several things required to make a “valley” installation waterproof. 

There are two ways to install valley shingles: 1) “open cut”, and 2) “closed cut”.

Here’s an illustration of each: 





http://esgreenville.com/wp-content/uploads/2011/07/timberlineinstallation.pdf

You have the “closed cut” system.