In a number of chess variants listed on chessalt.com, there is mention of 'Focus pieces'. What are they?

They are these stacking counters or, more specifically, EAI Education Stacking Counters. (A set of 500 in their various colors is found on Amazon for about $23.) I have found them, along with smaller than standard-sized chessmen, to be very useful chess variants accessories. Stacked too high, they are prone to topple. But they do loosely lock into one another and stack admirably. Something with a wider base has been suggested. But I think they serve pretty well.

I call them Focus pieces because that is why I first got them, in order to make my own game board and pieces for Sid Sackson's excellent game Focus, a game described as checkers on steroids. Focus is an elegantly simple yet deeply strategic game. I would recommend Sid Sackson's games Focus and also Acquire to anyone. In time, I would like to try more of his games and those he recommends in his book A Gamut of Games.