I heard references to "forward slip" to get rid of altitude on approach.

Both prior posts pretty much nailed it. A forward slip can be made either
to the left or right. Basically, you intentionally use cross controls
(rudder & aeleron). I believe that a forward slip generally refers to using
a slip to keep the aircraft aligned with the runway for landing in a
crosswind. This could mean landing on one wheel to maintain directional
orientation. This is my preferred approach when flying in a cross wind in a
taildragger.

A side slip is similar but in this scenario, the slip is can be more
aggressive. A side slip is used to shed altitude.

When practicing slips, be careful. Remember, you are intentionally using
cross-controls near the ground where inadvertant spin recovery could be
impossible.

I got my call sign "slip'er" because I thought that they were so fun I
always come in high and slip it in.