Hit frequency measures how often a spin pays anything at all. A high hit frequency means frequent small wins, and a low one means long gaps between wins that are usually bigger. It says nothing about how much is paid back overall. Two slots can have the same hit frequency and very different return to player and volatility. A win that pays less than the bet still counts as a hit, even though the player loses money on that spin.
Example
A slot with a hit frequency of 25% pays something on about 1 spin in 4. Many of those wins may be smaller than the amount bet.