In its most basic form, it tries to understand your question, rephrase it to be better for a Google search, searches, picks a handful of results and summarises it for you with GPT-3.5 (I think turbo? not really sure).
If you use their Copilot feature, it gets much better. It will try to understand your question, split that into multiple focused Google searches, picks a lot of results (sometimes like 25) and condense them into a summary with GPT-4 (or any top model you wish to use). That summary can get lengthy (I've got 2-page ones) as your query gets more complicated. It will also sometimes ask follow-up questions to clearify up your query. You get 5 uses every 4 hours without paying, and something like 600 uses a day if you pay.