The nice thing about For Honor is that it's multiplayer, meaning that even if you totally suck you have a whole squad of soldiers who are there to help you out — even if you die. Should you find yourself in the position to revive someone else, though, you should probably know what you're doing.
For Honor Open Beta: How to revive a fallen teammate
If someone on your team dies, you can revive them by walking up to them and holding circle on PS4, B on Xbox One or E on PC. However, you cannot revive teammates that have been executed. You'll know whether or not you can revive a teammate based on the icon above their sad, floppy corpse.
If there's a cross-shaped icon, you can revive them. If there's a skull icon, that means they're permanently down for the count — and probably headless. There's just no coming back from that.