In video games, a skill ceiling is the limit of ability; how good you could theoretically get at the game. The game of rock, paper, scissors has a relatively low skill ceiling. Short of mind-reading (or learning individual habits and tells; although there’s only so far you can go with that), there isn’t much a player can do to get better. Rock paper scissors also has a low skill floor, although these two levels are often independent of each other.
A skill ceiling can be defined by what’s possible within the rules of the game or even by human physiology. For example, there’s only so many buttons a human can press per minute. In practice, there are also soft skill ceilings - situations in which a player could continue to improve, but in which there are rapidly diminishing returns to doing so. high skill ceilings allow for skill expression.