Companies constantly add and remove shares: buybacks pull shares out of circulation, while new issues, mostly shares handed to employees as pay, add them. Net share change is what is left after the two cancel out. It matters because your ownership is a slice of the total. Picture a company as a pizza where you hold one slice of ten: cut the pizza into fewer slices and your slice grows; cut it into more and your slice shrinks, even though you did nothing. A negative number means the share count shrank and every owner's stake automatically grew. A positive number means dilution: your slice got thinner.
Watched over five years, this is one of the clearest windows into how management treats its owners. Companies that steadily shrink their share count, Apple and AutoZone are famous examples, reward shareholders through buybacks alone, before any dividend or growth.
The worst mix is the reverse: a big stock-based-pay bill alongside a rising share count, because owners pay twice, once as an expense that lowers profit, and again as dilution that thins their stake.