This is the 330th most frequent Japanese word.
チェーン
"Chain."
In this context, 'チェーン' is used to mean a 'mechanical chain,' particularly the one that drives the bicycle.
彼は自転車のチェーンを直しています。
He is fixing the bicycle chain.
Here, 'チェーン' denotes a 'security chain' used on a door for extra safety.
家に入る際にドアのチェーンをかけました。
I latched the door chain when entering the house.
The term 'チェーン' here refers to a 'chain store,' meaning a series of retail outlets operating under the same brand.