What are "Landwalk Abilities"

For example

Hammerheim

Target creature loses all landwalk abilities until end of turn.

What does "Landwalk Ability" mean?

Any ability like Forestwalk, Mountainwalk, etc.

what does it do?

Landwalk means that if the defending player controls a land of that description, then the creature with that landwalk cannot be blocked.

Example: My creature has mountainwalk, and you control a Mountain. You cannot block my creature.

you mean opponents needs to have those lands or I do for it to work (or any player)? Because I have no control over what he puts in his deck? For example what if I use a creature with forestwalk in my green deck but other player is using a black deck?

The defending player needs to control the land. Let's say you're in a three player game. You have a creature with Forestwalk; opponent A controls a Forest, but opponent B doesn't. If you attack opponent A or a planeswalker he controls, he can't block it. If you attack opponent B or a planeswalker he controls, he can block it.