A referent is what a word or symbol stands for. The referent is the concrete thing that’s being referred to, so an actual chair would be the referent of the word chair.
The referent of words such as table or building is pretty clear, but some referents are more abstract, like for the words happiness or danger. The word referent is handy in grammar-land, when you’re trying to figure out how words are being used in a sentence. Also, Frankenstein’s monster could be the referent of monster, experiment gone wrong, or big green guy.