Can someone please tell me how do you use this feature? I assume it is to match up the spacing on two edges, but whenever I try it it says the edges don't match

The edges must share a common vertex. The function takes the first spacing from the source edgee (first edge selected) and applies it to the destination edge (second edge selected). (The two edges must be selected relatively close to the common vertex.) This is a one-time match - any later changes to first cell size do not persist.