A player that takes enough damage is incapacitated, and can no longer take damage. That player can still communicate over Voice or Text chat until they are secured, and can be revived by friendly players.
Takedowns and Melee
An enemy player can be neutralized from behind by performing a Take-down, Left [Ctrl] key by default. Take-downs do not alert nearby enemies with sound, though the victim may still use Voice chat to alert nearby teammates.
Reviving Friendly Players
Reviving requires a friendly player to move close enough to interact with the incapacitated player and hold the interact key ([E] by default) for ~2 consecutive seconds. A player cannot be revived more than twice, and will be revived with less health if that player was neutralized by a shot to the head.
Securing Enemy Players
Secure incapacitated enemies using the interact key ([E] by default). Secured players are removed from the round after 5 seconds, cannot communicate by in-game voice or text chat to their team, and are restricted to spectating their teammates in first-person (by default). They can, however, still be revived within the 5 second window.