In autoprotolysis, a proton is transferred between two identical molecules, one of which acts as a Bronsted acid, releasing a proton which is accepted by the other molecules acting as a Bronsted base.
Auto-protolysis of water is self-ionisation of water.
The auto-protolysis (self-ionization) of water takes place as follows:
H2O(l)+H2O(l)⇌H2O+(aq)+OH−(aq)
The significance of auto-protolysis is that it shows the amphoteric nature of water i.e., water acting as acid as well as base.