Perplexity is a measure of how certain the model is of the next token. It's calculated by looking at the probabilities that the model calculates for the next token in a stream. If there are several choices for the next token with similar probabilities, that's telling you that the model is having a hard time telling what the right answer should be: the model is more perplexed, perplexity is higher. If there's a single option with a much higher implied probability than any other, that means the model is more certain, and perplexity is lower.

Note that this has nothing to do with whether the answer is objectively correct. It's just measuring how confident it is.