Hit frequency is the proportion of spins that land any winning combination. A hit frequency of 25% means, in the long run, roughly one spin in four pays something.
It is a different thing from RTP. A game can hit often and still return little, because many of those wins are smaller than the stake. Those are losses disguised as wins: the reels celebrate, but your balance has gone down. A spin that pays nothing is a dead spin.
Hit frequency is one ingredient of volatility. Low-volatility games usually have a higher hit frequency and smaller average wins; high-volatility games hit less often and save more of their return for rare big results.
Not every studio publishes the figure. Where it is given, it normally appears in the game's help screen alongside the RTP.