I went on a date last night with a girl from the zoo...
I went on a date last night with a girl from the zoo. It was great. She’s a keeper.
This is a pun. The humor in this joke centers around the word keeper.
First, there is an occupation "zookeeper", which is someone that tends animals at the zoo. Also, there is a colloquial phrase "(s)he's a keeper", which means that the person in question would be a good person to marry.
Both of these could reasonably be said in this context.