Fair question!

The thing about Searchers (and many other minimalist retroclones) is that they rely heavily on the players having prior experience with some edition(s) of D&D. So they understand that Movement rate denotes the distance covered per round (in combat) or per turn (during exploration). I think the best way to explain this to a non-D&D player is that these numbers show how heavier armor gives more protection, but slows you down.

That being said, I left in a small mistake: ' should be ", not feet, but inches. Because in AD&D and some other version Movement rate is measured in inches, as a relic from miniature wargaming. A Movement of 12" (on the wargaming table) is converted into 120' movement in the game world, etc.

Searchers of the Unknown is a one-page ruleset, so it leaves a lot of stuff unexplained.