Landwalk is an evasion ability. A creature with landwalk can't be blocked as long as the defending player controls a land of the specified kind. The kind is named before the word: for example, swampwalk, forestwalk, islandwalk, mountainwalk, plainswalk, or broader types like snow landwalk, nonbasic landwalk, or legendary landwalk. The reminder text spells it out: if the defending player controls at least one land that matches, the creature is unblockable; otherwise, it can be blocked normally.
In play, landwalk is a conditional threat. It costs you nothing to use and checks the board automatically, so it punishes opponents who happen to have the right land type. Against decks without that land, it does nothing. Broader versions like nonbasic or legendary landwalk are harder to avoid because many decks run those lands. This makes landwalk a flavorful way to reward metagame awareness.
