It`s the very first fitting you see at the tank.It`s function is to prevent gas from "free flowing" out of the tank if a line ruptures.Just a spring and ball setup.
Technically you only need one if the following is not met.
183.568 - Anti-siphon protection
Each fuel line from the fuel tank to the fuel inlet connection on the carburetor must:
(a) Be above the level of the tank top
Even if every part of the fuel system is all above the tank it's still not a bad idea to have one installed. They're cheap and you can get one form both Boaters World and West Marine.