Question

what do u mean by self ionization of water

Answer 1

when we pass electric current to the water by using anode and cathode it is self ionised into h2 and o2
may be that is called as self ionization........

Answer 2

Self ionisation of water means water itself reacts with water and breaks into its constituent ions.

$\sf{H_2O + H_2O \rightarrow H_3O^+ + OH^-}$