Your pickups are a coil of wire surrounding a permanent magnet which projects a magnetic field in the area of the strings. The strings are at least partly made of ferrous metal, so when you play, they upset this magnetic field. The changing magnetic field induces a voltage in the coil, which is the output signal from the pickups.
The pickup coil not only picks up changes in magnetic field, it will pick up any other electromagnetic fields as well, which includes electrical noise from dimmers, power supplies, computers, etc.
To reduce extraneous noise, you shield the electronics to try to prevent EM fields from getting to your pickups. One important part of this is to shield the electronics cavity. By making a conductive cage around the electronics in your guitar, you can block out weaker interference. Making this cage is usually done by using metal or conductive paint all around the control cavity and in the pickup cavities.
The better the shielding, the more noise you can eliminate. In guitars, the bridge is often attached to the grounded shield. This makes the strings act as part of the shield which blocks off more EM from the pickups. But when you touch these grounded strings, YOU become part of the shielding as well. Since you're big and filled with water, you are a much more effective shield than the measures you've taken on your guitar.
To have the shielding work, you want to completely surround the electronics. Every part of the shield needs to be connected to a common point electrically, and that point needs to be connected to your signal ground. I'm not exactly sure what a jack plate is (I assume that this is a plate that the jack is mounted on, like on a Strat or Les Paul), but you want this to be part of the grounded stuff.
On a guitar like a Strat, you want to shield the bottom of the pickguard, but you also need to make sure that the foil shielding there is in full electrical contact with the rest of the grounded stuff. If it's not a zero-resistance connection, then you will compromise the shield effectiveness, but it will still work to some extent. The best kind of grounding be what's called star grounding where a wire connects every piece of the shield to a common point. Obviously having your back plate or pick guard connected to the electronics compartment by a wire is a hassle for maintenance.
While we're on the subject, you make a good ground for your guitar, but you do so at some risk. A regular electric guitar has an unbalanced signal which is carried on a single conductor and on the shield braid in your cable (versus the balanced signal of a microphone where the signal is carried on two conductors in the cable and the shield is a separate line). In the amp, the signal ground and the electrical ground are usually connected together.
If there's a wiring problem of your amp relative to other AC-powered stuff on stage, you run the risk that the signal/power ground might actually be connected to the AC live power. Since the guitar's strings are grounded and you are touching them, you become the path for full line power if you touch something which has opposite ground - really unpleasant or even fatal if that turns out to be a microphone. These days, shock hazard is reduced by 3-prong grounding, but there are lots of opportunities to play in places where the outlets might be wired incorrectly or the ground line might not be connected. If you've every used an old tube Fender Bassman or the like, you are certainly familiar with the death switch which reverses the live and ground lines! When you use an amp like this you have a 50% chance of getting zapped, depending on how everything else on stage happened to be set up!
There are ways to avoid the shock hazard, but most of them compromise your shielding. On guitars equipped with active EMG pickups, they recommend that you NOT ground the strings for safety. The reason that this can work is that the EMG active internal construction is designed to reduce hum (they are all hum-cancelling designs) and the electronics module inside the pickup itself is heavily shielded.
David Fung
