An ambush gig is a stand-up comedy gig where the crowd didn’t come expecting to see stand-up comedy.
The most common case is a pub that has decided it wants to put on a comedy night, but doesn’t realise that comedy needs a separate room to work. So you’ll be performing in the main bar, to a crowd who mostly showed up to socialise with their friends, trying to talk over music and the sounds of fruit machines, in front of a TV silently showing an ongoing terror attack.
Another type of ambush gig is when someone organising an event like a wedding or a birthday party decides to book a comedian, even though the guests aren’t expecting to see stand-up comedy and many of them don’t like it.
Even if the crowd sits politely and listens, they probably won’t be very responsive.
Don’t try to do stand-up comedy to people without their consent. Don’t do ambush gigs.
My advice based on bitter experience is that if you get to a gig and realise it’s an ambush gig, just walk - unless you have a really good reason to stay.