“Sundries” are “items not important enough to mention” and represent things you totally remembered to buy back in the last town. You have, by default, 50 GP work of sundries.
Sundries have to be cheap items that can fit in your backpack and aren’t specific or magical. This allows players to not have to worry about keeping track of things like torches, rations, a sleeping bag, chalk, etc and allows inventory to be a little more flexible (“Of COURSE I bought a climbing pick, I'm a seasoned adventurer… did you REALLY think I’d forget that?”)
When you are in a town you can spend GP to buy an unspecified amount of Sundries. As you use these Sundries, subtract the total from however much you spent on Sundries. You can refill this total in any town with a market but only up to your maximum value of sundries (by default, 50 GP). If you lose your backpack (or whatever you use to carry things) you lose your sundries.
A GM has discretion over what can be produced, particularly if it stretches beyond what is reasonable to carry.
You can either spend money to refill your reserves of Sundries or you can use treasure to do so. A GM may have “non-descript treasure” or piles of “Sundries” with a certain gold value that the players can use to refill their stores of Sundries.
Because Sundries are part of the game a GM can comfortably assume that players have access to most pieces of small adventuring gear. This shifts the design away from “did the players pre-plan” to “how can the players creatively solve this problem”. In other words you don’t need to worry if players brought ice picks or warm blankets on their arctic expedition and should focus on challenges like “are the players smart enough to use ice picks to help them climb the ice wall”.