安全带(ān quán dài) - seat belt in Mandarin

安全带 (ānquándài) is a "seat belt". 安全 (ānquán) means "safe, safety" and 带 (dài) here means "belt, strap", so it is literally a "safety belt", the strap that keeps you in place in a car or on a plane.

The verb that goes with it is 系 (jì), "to fasten, to tie": 系安全带 (jì ānquándài), "to fasten your seat belt", is the standard phrase, whether it's a driver's own instruction to a passenger or the announcement you hear before a flight takes off.

Note the pronunciation of 系 (jì) here: as "to fasten", it is read jì, not the more common xì you'll meet in words like 关系 (guānxì), "relationship". Getting the tone and reading right on 系安全带 (jì ānquándài) marks you out as someone who has actually listened to native speech rather than just read the characters.