When you start off a conversation with "Listen", it seems that you want to get to the main point of the conversation quickly. You use it like this:

Listen, I know that we've had disagreements in the past, but I hope that we can work together on this.

Listen, we don't have much time, so let me explain what happened.

There are also other reasons to start off a conversation with "Listen" as well, such as when you're giving advice or when you're introducing an uncomfortable topic.