A Big Crunch is a possible endstate of an initially expanding universe where the scale factor reaches to zero in finite time after the universe stops expanding and begins contracting.
One way this can occur is a universe filled with a field that has an energy density that dilutes faster than dark energy (i.e. energy density increases with scale factor). Fields that do so include matter, radiation, and ultralight. For a universe dominated with a field with initial energy density that varies with scale factor as and minimal curvature (for a 3D realmverse: if , the universe is domain wall-dominated; if , the universe is cosmic string-dominated; if , the universe is matter-dominated; if , the universe is radiation-dominated; and if , the universe is ultralight-dominated), the Friedmann equation is and therefore the scale factor as a function of time is . This reaches zero at time .
To an observer inside the universe, the Big Crunch represents the end of everything - no force is able to keep structures apart, so galaxies, solar systems, atoms, and eventually spacetime itself are crushed together and the observer's own observable universe will be compressed into an infinitesimally small point reminiscent to how it was during the universe's beginning.
Following a Big Crunch, it is possible that the collapsed universe’s spacetime could re-expand and a new universe can potentially be born in a Big Bang event after the previous universe underwent its Big Crunch event. An event like this is known as a Big Bounce.
It is likely that our universe will not end in a Big Crunch event as its expansion has been observed to be accelerating. This suggests that a Big Rip or heat death event is far more likely to occur in the future.