The Halo series has always dealt with death in varying ways. You lost your shields, then your lost your health and then you died before respawning at the nearest checkpoint. During co-op matches you had to wait until your friends were out of the firefight before you could spawn on their location.
It was all very artificial and "game-y," with the idea being that in solo play at least the "real" playthrough is the one in which Master Chief never fell to enemy fire.
All that is over in Halo 5: Guardians.
You're fighting alongside three other Spartans, even in the single-player campaign. You always have computer-controlled NPCs along for the ride, and they can drive vehicles, help out with fighting the enemy and of course they're there to revive you if you fall.
Which is a huge change, at least in the first few missions of the game I've played, and am allowed to talk about. It's very hard to die and stay dead in Halo 5; calling for a heal from your teammate, be they your real-world friends or the computer, is just a button-press away. They still have to fight to your location and survive long enough to get the heal off — the process takes a few seconds — but it's rare to run into a situation where this won't happen.
This is on the Normal difficulty setting, by the way. If you increase things you can and will die rather often if you don't take care of your positioning. You'll just have to watch your computer-controlled team run towards you to be cut down while your timer runs out. It's actually pretty brutal. Keep in mind there is no "right" or "wrong" difficulty; it's been said that Heroic is what's "intended" for prior games, normal difficulty is said to be for people who know first-person shooters in genera, while Heroic is described as being designed for "veteran Halo players."
So it's very unlikely you'll die unless you're playing on Heroic or harder, and that changes the feel of the game significantly. The lack of spawning at checkpoints when you're cut down means that it can be a simple thing to shoot through large groups of enemies just by taking a few out, waiting to be healed, taking a few out, waiting to be healed, etc. It's not a fun way to play, but you can grind your way through using this method.
Also, "dying," which is in this context means being cut down and having to wait for that heal, is painful. It sucks to be forced to just sit there tapping on the button to call someone over while the battle goes on around you.
It doesn't take very long for someone to get to you in most situations, but it kills the rhythm of the game, and in those circumstances where you're too close to a powerful enemy for anyone to get to you it can take an annoyingly long time before your time runs out and you "die" for you. Even though the mechanism has changed, you're going to want to avoid dying as much as possible.
This change is interesting mechanically, especially since it will change co-op play now that players don't have to escape firefights to get fallen teammates to spawn, but it also alters how the game feels on a deeper level. There was always a sense of isolation to Halo games, the idea that Master Chief was fighting by himself against impossible odds. He was his own backup, and that sense of being apart from the other soldiers was part of what made the world feel so strange and off-putting. It was you vs. them, and "them" was everyone else.
Now you're part of a team, even when you're playing as Chief, and at least in the opening levels they will help you up if you fall. I'm not saying it's great, I'm not saying it's terrible, but it's a major departure for the series.