system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 How do "Lock Ons" work in military jets.
gpt
"Locking on" is a very general term; there are a bunch of different ways you can track a target (which is the goal of all of them).

Simplest is passive tracking: identifying and following the target by looking for it against the background, for example looking out for a cyclist on the road. They don't have any lights or reflectors and you don't have a torch, so you just have to look closely and rely on background light to see things. When you're "locked on" here, what you're really saying is that you're confident you can keep track of the cyclist. This is how daylight TV-guided missiles work: they have a sensor which detects contrast (or in more advanced systems, shape and colour and contrast and...) just like we do. Very similar is passive tracking of emissions: just the same as before, but the cyclist has lights now. You can see the light they emit much more easily, so you can a) find them further away and b) track them better, because they are more visible. This is what the Sidewinder does (with infrared light from jet engines, and in more advanced versions from the heat on the leading edges of wings and things), and the HARM (an anti-radar missile for killing surface-to-air missile sites) does with RADAR emissions. When all these things (say they) are "locked on", it just means their software is confident that they can track the thing they've been assigned and not get confused. It's a sliding scale, essentially. 

More complicated (and more binary) is "locking on" with an active radar or missile. Back to the cyclist: it's dark now, so to find the bike, you first use a big torch set to wide beam to look around, scanning a wide area. Once you spot the cyclist, you switch to a narrow beam pointed straight at him, and move it so you follow his motion. You're now "locked on" because you've switched from not actively following a target, to actively tracking their movements. Several different kinds of missile then use this: "beamriding" missiles have a detector in the *back* of the missile, and stay within the torchlight (or rather, radar beam) until they hit the target; "semi-active radar homing" does a clever trick where the missile follows the reflection from the target independently. You keep the target in the beam, and the missile follows the now-quite-bright target - equivalent to your mate on another bicycle hunting the first guy, by looking where you shine the torch. Locking on isn't obligatory with either of these methods: you can (and early SAM operators had to) just sweep the beam manually, and manually track the target. Obviously this is pretty difficult against a target who doesn't want to die, and is a lot to ask of a modern fighter pilot, so we have computers for it now.

The most complicated variant is "active radar homing" missiles, which pack a radar transmitter and receiver of their own into the front of the missile. Rather than relying on you to keep the target lit up, your mate on the bicycle now has his own torch, so once you've found the target cyclist, he can follow it for himself, as he closes in. You're free to start looking for other targets. "Locked on" here means that the missile's radar has acquired the target from the plane's radar and can track it, rather than just that the plane's radar is tracking.

**EDIT** (for Americans and others who rejected our fair and rainy isle) : "torch" means "flashlight". In some parts of Yorkshire many people's torches actually have a small coal fire behind the lens to provide the light, and young boys are often employed in winter to stoke people's torches up when they leave the pub.