Every planet sits at the bottom of a gravity well. To leave, a probe has to climb out — and climbing costs speed. Kinetic energy pours into potential energy on the way up, and pours back on the way down.
The total of the two — the mechanical energy — never changes along a coasting arc. Physicists call this conservation of energy the most powerful accounting rule in all of science: you can move energy between the books, but you can never make it appear or vanish.
There is exactly one launch speed where the total reaches zero. Below it the probe always falls back; at or above it the well can't hold on. That threshold is escape velocity — about 11.2 km/s at Earth's surface.
It's the same sum Voyager 1 balanced in 1977: given just enough energy, plus a gravity assist from Jupiter, it climbed out of the Sun's well for good and is still coasting, decades later, on an account that has never once gone out of balance.
Something in the simulation stopped unexpectedly — the lesson continues without it. Nothing you did was wrong; you can move on.