In broad strokes, it's the same as post-modernism in the rest of the arts and humanities: it questions whether "objective knowledge" is a concept that even makes sense, and whether any of the things we know correspond to something absolutely true about the world.

Hayden White is one of the names to know for this part of historiography. He wrote a book in the '80s called The Content of the Form: Narrative Discourse and Historical Representation, which argued that historians aren't saying anything about the world-- we're saying things about our sources. We're basically reading texts and interpreting them within our own cultural context, which means we're not doing anything different than scholars of literature, and we're certainly not making any objective knowledge about the past.

This was a very threatening idea to a lot of contemporary historians, which sparked a few kinds of reactions. One was an embrace of post-modern literary and art-historical methods in history. If you read historical books/articles that have words like "narrativity," "the image," or "constructing" (not about buildings), they are likely coming from this tradition. Another reaction was to use more and more quantitative methods and other methods borrowed from the "harder" social sciences, including economics. And a third was to question the underlying assumptions of the post-modernists and to say that of course contemporary historians are reading things as the contemporary humans we are, but that doesn't mean that nothing we say about the world is true.