Legendary landwalk is an evasion ability: a creature with it can't be blocked as long as the defending player controls a Legendary land. If they have one (like a Legendary land such as a uniquely-named land), your creature simply can't be blocked—no choice, no triggers, just unblockable against that opponent.
The nuance people miss: landwalk is conditional, checked when blockers are declared. It only matters per-player, so an opponent without a Legendary land can block normally. It also doesn't grant any other protection—your creature can still be killed by removal or combat tricks; it just gets through.
In Commander, this is extremely niche. Legendary lands aren't widely played, so it rarely turns on. It's mostly a curiosity rather than a reliable evasion strategy—don't build around it unless your meta runs lots of Legendary lands.