There's nothing worse than being stuck on a narrow path behind two people walking abreast of one another. If they only knew you were there, they'd walk single file so you could pass them. Abreast means side by side.
Abreast can also mean alongside. In a race, you watch the front runners, but it's also important to pay attention to who is coming up the side and abreast of the leader. They are the most likely to find it within themselves to sprint forward for the win.