A "relevant" fact or opinion is one that's related to the main topic of a conversation, or important for a certain group of people.
For example, if you're talking about the world economy, news about the stock market might be "relevant" to that topic. News about a new animated movie that was just released is "not relevant".
When someone tries to talk about something that is not related to what you think is the most important part of a conversation, you can say that their point is not relevant:
Whether you approve of the new president or not is not relevant. The fact is, the election is over and we have to move on.
You can use the phrase "relevant to ___" to name the topic:
The hard part about teaching is convincing the students that the stuff you're teaching them is really relevant to their lives.
You can also use the word "irrelevant", which means "not relevant".