Means full attention on your friend, to find empathy in what they're saying. It means less personalising, especially in your response. "Oh that's like what I'm going through with so and so..." - unless it's entirely relevant.
It means no distractions, like typing on your laptop or tidying your house while they open up over the phone, or texting while they talk at the table.
It means not laughing off what they're saying because you weren't prepared for a vulnerable conversation.
When there's genuine listening the other friend notices. The bond grows stronger because they're visible. Their words are valid. And there's trust.
You may not have answers or solutions, but listening is enough. Most of the time, it's the best support a friend can give.