Blistering
Small raised bumps that form when finishing seals the surface over trapped bleed water or air, which then collects underneath and lifts the skin.
What does blistering mean?
Blisters appear during or shortly after finishing and often break under later passes, leaving hollow spots. The root cause is premature closing of the surface, aggravated by air entrainment, sticky mixes, cool subgrade under warm concrete, and windy conditions that dry the top while the interior is still bleeding. Blisters and delamination are the same mechanism at different scales.
In practice
If a finisher hears a hollow sound under the trowel, the slab is blistering and continuing to close the surface will make it worse.