[playbox12]

It just how the function is laid out, return is basically just a value or variable. If you create your own function you could use return to send a certain variable back like with GetName it would be the players name. For the native functions in samp, it is controlled by the samp-server, on the wiki you can see what it does. OnPlayerTakeDamage doesn't handle returns according to the wiki, so it doesn't matter what you return there.

(It is basically just an exit out of the function, usally used to tell the script the status of which it returned with).