Crack sealing is cleaning out the cracks in an asphalt surface and filling them with a hot-applied rubberized sealant so water can't get into the base.
Water through a crack is how a surface problem becomes a base problem. It softens the base, freezes and expands, and turns into the potholes and alligatored areas that need patching or full-depth repair. Sealing cracks while they're still single lines is the cheapest pavement maintenance there is.
Some specs split the term. Crack sealing is for working cracks that open and close with temperature, often routed first so the sealant has a reservoir. Crack filling is for wider, mostly stable cracks. Ask which one the spec means before you price it.
It's priced by the linear foot, and the footage varies wildly between lots of the same size, so walk it and price it as its own line instead of folding it into the sealcoat rate. Alligator cracking, a web of connected cracks, can't be crack-sealed. Note it as a patch area.