Roshan, sometimes also referred to as Roshan the Immortal, is the most powerful neutral creep in Dota 2. It is the first unit which spawns, right as the match is loaded and is the only NPC which scales over time. During the early to mid game, he easily outmatches almost every hero in one vs one combat. Very few heroes can take him on alone during the mid game. Even in the late game, lots of heroes struggle fighting him one on one, since Roshan grows stronger as time passes.
Roshan is not a creep which can just be farmed like the other neutral creep camps, fighting him is an important team decision, as it needs the correct timing and approach since it can decide the future of the match. Usually, it is fought as a team when it is safe to do so, meaning when the enemy team does not display a threat at the moment (for example after a successful team fight).