Escape velocity is the speed at which something moving away from a planet or moon is going fast enough that gravity can slow it forever without ever quite stopping it. Below that speed, what goes up comes back down. At or above it, the object keeps going.
The number depends entirely on what you are leaving. Earth's escape velocity is about eleven kilometres per second — the reason launches here demand enormous, fuel-heavy rockets. The Moon's is roughly 2.4 kilometres per second, less than a quarter of that.
That gap is the whole argument for the Moon as a launch site. Leaving the Moon takes a fraction of the energy that leaving Earth does, which is what makes ideas like a fuel-free magnetic launcher physically plausible there and absurd here.
The phrase travels well beyond rocketry. You will meet it again in talk of "longevity escape velocity" — the same shape of idea, borrowed: a threshold past which you outrun the thing pulling you back.
