Truth. Alternate translations, ‘reality’ or ‘right/correct’. Haqq is the opposite of bāṭil, which means false/falsehood. 

‘Al-Haqq’ is one of the names and attributes of Allah. This can be simultaneously understood to mean that Allah embodies the concept of truth, that Allah’s will is always right/correct, and that Allah is reality, i.e. He created, sustains, and permeates all of existence.