Meetings don't actually go off-topic, people do. It usually takes at least two people adrift to take the meeting off-course. That means you don't have to be an expert on meetings to help. You just have to notice when certain things show up in a meeting's conversations.
Any topic can attract comments irrelevant to the meeting agenda or objective.
Juicy as they are, such comments seduce one or more participants. What to do. Great leader tip. If it's not important, other participants will confirm your assessment and those engaged in the conversation can be asked to continue off-line. Breaking time agreements.