The biggest neutral creep in a popular computer game called DoTa. Dota is a map of Warcraft 3 Frozen Frone, Roshan is a huge rock figure in the game and generally requires a few team mates to kill. When he dies he releases an item called Aegis of the Immortal which allows a player to come back to life as soon as they die. After you kill roshan he will respawn after 10 minutes. After you kill him three times he releases an item called Cheese which, instantly gives you a health and a mana boost.
"Lets Rosh" - Means the team will go kill Roshan
"Im gna kill Roshan" - Self explanatory
"They might be at Rosh" - The opposite team assuming their enemy is at Roshan's location