吓 + 了 + 一 + 跳
While 跳 typically means "to jump," it also appears as a noun in the fixed expression 吓了一跳 (xià le yī tiào), meaning "to get a fright" or "to be startled." In this usage, 一跳 functions as a complement describing the involuntary jolt of surprise. This is one of the most common idiomatic uses of 跳 for beginners.
我打开门的时候吓了一跳,没想到你在。
Wǒ dǎkāi mén de shíhou xià le yī tiào, méi xiǎngdào nǐ zài.
I got a fright when I opened the door — I didn't expect you to be there.
半夜听到奇怪的声音,我吓了一跳。
Bànyè tīng dào qíguài de shēngyīn, wǒ xià le yī tiào.
I got a fright hearing a strange sound in the middle of the night.
她看到成绩的时候吓了一跳。
Tā kàn dào chéngjì de shíhou xià le yī tiào.
She got a fright when she saw her grades.