"It's not relevant." or "That's off-topic."
It is used to indicate that something being said or discussed is irrelevant or unrelated to the current topic.
Example:
* Person A: "I'm really upset because I lost my phone."
* Person B: "Well, at least you have a new car."
* Person A: "Pas le rapport!"
Here, Person A is saying that Person B's comment about the car is irrelevant to the fact that Person A lost their phone.
In essence, "Pas le rapport" is a way of saying "That's not the point" or "Let's stay on topic."