A mixture of experts model is built from many smaller sub-models, each a bit of a specialist, plus a traffic director that routes each question to just the right few. Because only a slice of the full model activates for any given task, you get the smarts of a very large model without paying to run all of it every time. It is a clever way to make big models faster and cheaper.
For example, A coding question might be routed to the parts of the model best at programming, leaving the rest idle.