在means: 1)in,(with place names) 在 2)by,by day/night (during) 在白天/晚上 3)with,(together with，at the house of) 和…在一起 4)be,(talking about place) 在 再means:again再见,more再一个(one more)

在家，在学校，在星期一 at ,on 再一次 once again

“在“means a state , for example : now l am in shanghai , “再”is a word for the furture , just like the english word "again", there is a sentence maybe can help you undstand it , hope to see you again.

simply,"在" means "at, in, from "etc. this kind of preposition in english, and "再" means "again", pronunciation are same, but the meaning are much more different

在： 意思多用于一个人在那里，是名词 再：表示人们之间分别的动作，多用于动词