Used as an insult in online gaming to describe a player who attempts to use strategy, tactics, maneuvers, plays, etc. well beyond their own skill level (adj. "trying hard"). These players will often watch pro gaming streams and YouTube Let's Plays and attempt to copycat the person playing despite not having enough skill or technical prowess to be able to do so effectively.
Tryhards are not to be confused with bad players. The primary distinction is that tryhards will respond negatively to gameplay suggestions or criticism, and will frequently lash out against anybody who attempts to stray outside the established metagame.
Player 1: "I can't believe you picked that guy! He's a shit-tier hero! You suck!"
Player 2: "Calm down, I picked this guy because I like playing him. You should focus more on the game and less on me."
Player 1: "You're just a noob, lol. Everybody knows that pro players never pick that hero. Learn to play."
Player 2: "You're doing terrible this game, you fucking tryhard. You're the one who needs to learn to play!"