jet 是喷气飞机，lag 是时间间隔/耽搁。合在一起，就表达了由飞行产生的“时差”、“时差反应”或“倒时差”的意思。

(倒)时差, when traveling between different time zones, like from China to USA

It means the physical effects of going across multiple time zones and being in a different time than you are used to . Like traveling in airplanes across the word will cause jet lag, because your biological clock is not matching to the new time yet. So you want to sleep in the day and stay awake at night, it usually goes away after a few days in the new timezone.

It is when you spend a long time on a plane, and particularly when you change time zones. Your body gets confused, as it thinks it is in the old time zone, but you are trying to adjust to the new time zone. So, you might wake up at 4am, and be sleepy at 6pm. Generally, it has similar symptoms to being very tired. Not drinking enough water on the flight can also make the effect worse.