A pub meeting (also known as a bar meeting) is just that: a meeting held in a pub or bar. Said meeting is for 2 or more people to discuss anything vaguely serious and is not a social event.
However, due to the location and available alcohol, it often turns into a social event. For this reason, pub meetings are not hugely effective or efficient and not recommended if you want to get things done, genuinely discuss something that you are serious about or discuss business matters. They are best organised to discuss ideas in the form of a simple brainstorming session; and the addition of alcohol makes it that much more interesting.
We need ideas for that film we're gonna make. Let's have a pub meeting.