You are probably wondering what an HM is. HM stands for Hidden Machine.

These special items are very different from TMs.

There are only 5 HMs in the entire game of Pokémon.

They are reusable, and cannot be deleted from the pokémon's moves.

This means that, if you use HM 1 to teach Ivysaur Cut, you will not be able to teach him another move in the place of that move. You could also teach it to him in all four places. I don't know why you would want to do that, but you could. Then you could go and teach it to all of your other grass pokémon (I don't know why, but again, you could).