Low-paying symbols are the most frequent regular symbols on a slot and pay the least for a win. On many video slots they are the card ranks 10, J, Q, K and A, sometimes called royals; other games use gems or simple icons.
Because they land often, they generate most of the small, frequent wins in the base game. Many of those wins are smaller than the stake on games with lots of lines or ways, which makes low symbols the usual source of losses disguised as wins.
Their place in the maths is to keep the hit frequency up and the bigger prizes rare. Features often target them: symbol upgrades convert them into high-paying symbols, and some bonus rounds remove them to concentrate the reels on premiums.
The exact values are listed in the game's paytable.