Hit rate is most often used to mean how frequently a slot's bonus feature triggers, quoted as an average such as "one in 200 spins". It is sometimes used loosely for overall hit frequency, so check which one a source means.
A bonus hit rate is a long-run average, not a schedule. With an illustrative one-in-200 rate, some players will trigger the bonus round twice in ten spins and others will go 800 spins without it. Nothing builds up between spins, so a long gap does not make the feature more likely next time.
Games that pay much of their RTP through the bonus tend to have rare triggers and a thin base game, which pushes volatility up. Where a studio publishes the bonus frequency, it is in the game's help or information screen.