What's the "Strong" Side and the "Weak" Side, and how does that affect where I put my stuff? I've been told to put my mags on my strong side, by I have absolutely no idea what that means, or where it is.
It's the place most easily accessible to whatever hand you have available. So if you're right handed and you need to reload, it would be best to have your mags on your left side because you will most likely be holding the rifle in your right hand.
Blitz pretty much explained it. It would be inconvenient to place your mags on your right side if you're right handed, and on your left side if you are left handed. This statement was probably obvious, but I didn't think it would hurt to say.
But note that your left side is your weak side if you're right handed. Your strong side is whichever side your dominant hand is on. What you want to do is match the type of things you access with your strong and weak hands with their location on your body. For instance, when you reload a rifle, you support it with your strong hand and grab a fresh mag with your weak hand. That means you want to put your mags on your weak side (or center, which is easily accessible with either hand. You draw your pistol with your strong hand, so you generally want to place it on your strong side, usually your thigh or hip.
If you consider your strong side and weak side when you set up your gear, and you'll have faster and easier access to everything than if you just put stuff wherever.