A bar held up in the air, and one person running at it. Everyone takes the same height; then the bar goes up again. You stay in as long as you can get over it, and the moment you cannot, you are out.
可以想成一根橫在空中的桿子，和一個朝它跑過去的人。所有人跳同一個高度，然後桿子再升高一點。你過得去就留下，過不去就結束。整個項目的重點不是跳得多遠，而是能撐到多高。
From the word’s history
The noun high jumper is formed from high jump plus -er, the ending English uses for the person who does something.
high jumper 由 high jump 加上字尾 -er 組成，-er 指「做這件事的人」，所以字面就是「跳高的那個人」。
Wiktionary, "high jumper"