Ad-hoc meetings are one-off events that don't fit within any of your preset meeting types.
Say you have a meeting type (/demo) set up for exactly how you typically have demo calls, and that accounts for the majority of your meetings. It’s 30 minutes with your zoom information, has no specific date or time constraint, and it's set to be titled a certain way.
BUT now you have an emergency meeting that this meeting type won’t fully cover. You need a 2 hour zoom call tomorrow with the product team and a customer to talk about feedback on a new feature, and it should be titled “Sync on XX Feature” so everyone is on the same page. That is an ad-hoc meeting!