Statcast defines a 'hard-hit ball' as one hit with an exit velocity of 95 mph or higher, and a player's "hard-hit rate" is simply showing the percentage of batted balls that were hit at 95 mph or more.
Why 95 mph? Because, as the image below shows, that's when exit velocity begins to "matter." Another way of saying that is that balls hit at 40 mph or 70 mph will affect your average exit velocity differently, but in terms of outcomes, they're just two varieties of weakly hit balls. For true production, you need to get to 95 mph.
You can see the value when you look at the 2018 MLB outcomes for hard-hit balls (95 mph+) and weakly-hit balls (below 95 mph).
.524 BA, 1.047 SLG, .653 wOBA
Weakly hit balls
.219 BA, .259 SLG, .206 wOBA