A friendly is a creature in Team Fortress 2 that doesn't harm other players unless threatened. They usually stay in corners away from the action and may form groups. Typical behaviour is to do taunts like the Conga, Mannrobics, and Katzosky Kick. Friendly heavies often throw their lunchbox items to enemies.
Friendlies try to communicate with the enemy team to show that they are friendly. This includes crouch walking towards an enemy or crouching in a corner. They also spam the silliest voice lines for their character. They can also engage in a taunt that keeps them in the same spot for a long period of time.
Many friendlies make themselves very vulnerable to death to make it obvious that they do not want to attack. Many do it by holding melee weapons out, or by holding items that cannot do damage, such as the Sandvich. Spycrabs go even further by holding out their disguise kit and facing directly upwards, making them incredibly easy to kill.
When killed, friendlies usually become incredibly aggravated and will often call the enemy player "toxic", "tryhard", "cheater" ect.
Impact on gameplay.
Friendlies usually do not play for the objective and are a form of griefer (players who intentionally irritates players and do not play the game as intended). Friendly or 'Silly' servers do exist where large number of friendly players may be found. Oftentimes, players will go out of their way to kill friendly players instead of playing the objective.