Why is prepping necessary?
Natural fibers (e.g. cotton, bamboo, and hemp) contain natural oils. These oils coat the fibers and repel moisture, which is the exact opposite of what you want them to do. Prepping washes away these oils, increasing the absorbency of the diapers.
What materials need to be prepped?
Any natural fibers (bamboo, hemp, cotton) need to be prepped before using. Any manmade fibers (microfiber, stay-dry fabrics etc) do not need to be prepped as they do not contain the oils. They just need to be washed once to be ready for use.
How do I prep?
Prepping is easy - you just need to wash the diapers (or inserts) 3-5 times. They will gain peak absorbency after about 10 washes, but 3-5 is the amount generally recommended. Use whatever detergent you are planning to use on your diapers, and wash in hot. You don't need to dry them in between washes.
Any dark colored covers should be washed separately from light ones for a couple of washes to make sure the colors don't bleed. It is unlikely that they will, but just to be safe!
For the first few washes, keep your natural fibers separate from synthetics (like microfiber). The oils that wash off of the natural fibers will stick to the synthetic fibers and may cause them to start repelling, exactly what you want to avoid. After these first few washes, they can be washed together.
If you are prepping just a couple of diapers, you can wash them with other laundry as long as you aren't using fabric softeners.
Can I boil my diapers to prep?
Yes and no. Yes, it does work to boil for a few minutes instead of repeated washing, but don't do it for anything with snaps, PUL or elastics as you will ruin them. Cotton prefolds can be boiled to get rid of the natural oils, but I much prefer the washing method. Boiling is surprisingly hard work, and I am currently banned from my kitchen for a burnt broccoli incident, so I'll just stick to the laundry room.
What will happen if I use a diaper before prepping it completely?
Don't worry - there won't be explosions or anything dramatic, and your diapers aren't ruined forever. The worst that will happen is they will leak as they won't be fully absorbent yet. If you can't wait until they are fully prepped to get that fluff on the bum, don't panic if you get leaks!