A coupler is a fitting designed to connect 2 pipes together into a longer pipe.
It is a socket-welded fitting (except with steel pipe, in which case it has female threads at both ends).

With the plastic variety, one can remove the ridge in the middle of the coupler, in a process similar to bushing modification. This yields a short length of tubing which slides over the length of the pipe, which is useful for certain breech loading methods.