A burble is the turbulent low-pressure area immediately behind a body moving through air. In freefall, that means the region directly above a belly-flying skydiver's back.
Why it has practical consequences
A pilot chute thrown into the burble may not catch clean air immediately, causing hesitation in deployment. This is why hand-deployed pilot chutes are thrown out to the side into undisturbed air rather than simply released.
In formation skydiving, flying directly above someone puts you in their burble, where the air is slower and lift is reduced — which can result in dropping onto them suddenly as you fall through it.
These pages explain what things are so the language makes sense. They are not instruction and cannot substitute for it — skydiving is taught in person by rated instructors at a drop zone, and anything involving equipment handling or emergency procedures belongs there rather than on a web page.