actually the complete sentence is 这两个人走路的时候，彼此距离很近。sometimes we omit some words，in this sentence，we omit“的时候”，we say“时”，it means “when“”

It could be understood like <when>

In written (and classical) Chinese, 时/時 is/was generally used by itself to indicate 'when'. In the vernacular/ spoken Mandarin, 的时候 is used. It sounds very weird to say 时 in spoken Chinese and you should generally avoid it.

时这两个人走路

Ok, thank you!