It means Nikkah is not valid anymore. It happens when 1 spouse leaves the fold of islam. Accept another religion, become an atheist. Men and women become halal for each other by making Allah their witness and accepting all the responsibilities imposed on both of them by islam.

Hence if someone leaves islam then automatically their nikkah won’t be valid

No mufti or maulana or anyone can faskh nikkah on his own.

In the past, many muftis have given such fatawas for trivial things like opposing a political party, refusing to believe in some particular thing relative to particular sect etc etc. bears no value and neither it invalidates the nikkah.

However, among Qadianis and Agha Khanis the Khalifa and Imam respectively can issue divorce decree on his own.