The characters combine literally to mean “scared (and as a result completed) one jump.” This creates a clear and descriptive image of someone being so startled that they physically jump.
While the reflex to jump when startled is universal, the commonality and descriptiveness of “吓了一跳” in everyday Chinese conversation is notable. It's a highly somatic expression, meaning it directly connects an emotion (fear) to a physical bodily reaction (jumping). In English, one might say “You startled me!” or “You made me jump!” While “You made me jump” is similar, “吓了一跳” is a self-contained phrase that describes the entire event from one's own perspective. It's less accusatory than “You scared me” and more descriptive of one's own reaction. Compare it to the English idiom “to jump out of one's skin.” They describe a similar feeling, but “吓了一跳” is a standard, non-idiomatic, everyday phrase used much more frequently than its English counterpart. It's the go-to expression for any “jump scare” moment.
“吓了一跳” is an extremely common, informal phrase used in daily conversation. It's almost always used to describe a reaction that has just happened.