My best guess, assuming that's a Honda (35-50hp?). is it's just a gasket designed to minimize wear/rattling of the plastic cover located right above that hose. Impossible to replace (in one piece, as designed) without pulling the engines mid section, which requires pulling the lower unit.
However, if you wanted to keep the engine in top notch condition, you could remove that plastic cover (not hard) and glue that gasket in place around the bottom of it, then reinstall. Or, I think a lot of people will just ignore the fact there USED to be a gasket there... The ones I've had apart are about 50/50 with/without that gasket. I don't reinstall them or get new ones if they are missing. That's me though...