The pipe fittings can be male (M), female (F) or male + (M+). A male fitting fits into a female fitting, and in the case of two male fittings, a female connector must be used.
It also happens that gluing a pipe into a female fitting is not possible (no space, poor condition...). In this case, fittings with a specific size that fit inside the pipe will be used. Tubs.fr calls this size M+ to borrow the American term "barb".
These M+ connectors can be either smooth or ribbed-barb. This type of fitting works best with small size hoses (especially 1" or less), and from the same manufacturer as the fitting concerned. If you have any doubt, it is better to use fittings with ribbed-barb ends.