A long time ago, doors used big keys and people could look through the hole where you put the key. So this just means "look through the hole where you put the key" so you can see into a house (or outside).

Thank you so much Winston & Jura for Helping me :) we do have the same habit done by those! actually this phrase came in a text I was translating! so thank you again, best regards

It means like someone wants to look and know other peoples lives. In our culture it is done by women old and curious. Like in all the countries))

A peeping tom peeps through a keyhole!