Replacing the cassette on my wife's around town bike - an old GF Marlin. Takes a 7 spd cassette.

Never replaced a cassette before, and was looking at the new one last night along with my other stuff that arrived yesterday.

So, why to the two smallest gears rotate somewhat freely in both directions, and what will eventually lock them into place? I imagine it will be become obvious when I actually do the replacement, but was just curious.