If your chickens only eat pellets, they can be fine with no grit. That is because pellets are made from ground-up ingredients, and come apart again when they get wet inside the chicken.
But if your chickens eat other foods (including treats), they need grit in their gizzards to grind up the food. Grit is little rocks. Sometimes chickens can get enough little rocks from the ground in their run, sometimes they cannot-- it depends on the what the ground is like in your area. Providing grit is easy enough and cheap enough that many people just do it, regardless of what kind of soil is available to their chickens, rather than spend more time thinking about the matter. (Not everyone does that, but some people do.)
