The past present, and future walk into a bar.

The past present, and future walk into a bar.
It was tense.

This is a joke about grammar. It doesn't really work - it seems like this joke is trying to be a pun, but they got it wrong. The past, present, and future are all tenses, but the joke uses the word tense, which is a different form of the word tenses. The other meaning of the word tense is to be tight or rigid because you are nervous.

The grammar of the sentence as it currently is works with adjective form of tense, but to make it work with the noun form it should perhaps say "they were tenses". If it did this, however then it wouldn't work with the adjective form.

Lame.