Aim assist is the general term used to describe several mechanics that help keep your aim on target. Originally, this concept was introduced to help make classic Halo more accessible, as the analog input of controllers adds a level of difficulty to aiming. PC players do not get all of the same features of aim assist, but they do get some assistance in landing their shots.
Different weapons have different aim assist values, including the angle around your reticle in which it will take hold, as well as the range.
As a visual aid, we recommend watching this video, as it will show a visual representation of how it works in-game.
AUTO AIM & RED RETICLE
Auto aim describes the physical pull of rounds fired, bringing them closer to target. For this to work, you have to be within a certain distance to your opponent. This is indicated in-game by the in-game reticle turning red. On PC, this is not available to players, with 343 stating that it is to prevent cheating. Many PC players feel this puts them at a significant disadvantage, as aim assist directly helps with accuracy.
While auto aim is powerful, it is not a guarantee. Having your bullets curve towards an enemy does not meant they will always hit. It is a common misconception that the red reticle means you will land your shots. This is not the case.
Magnetism both slows down your aim when you’re on target, and it subtly follows your opponents movement when aiming. It is not exact, but many players will aim using their movement to stay on target instead of adjusting their aim to help maintain accuracy. This function is only available to controller players.
With the addition of crossplay to both the MCC and Infinite, the strength of aim assist has been brought into the spotlight. Aim assist is a hotly debated topic, and even 343 has acknowledged that balance between the two input methods is something that needs to be looked into.