Hybrid / All-in-Two Diapers are a two piece diaper system, which consists of a reusable waterproof shell/cover and an absorbent snap-in or lay-in insert. With an All-in-Two system, you can reuse the waterproof cover (as long as it's clean enough!) with a new insert at a future diaper change. This makes for an economical diapering system, as you don't need as many covers as you have inserts; Usually families find a ratio of 1 shell cover for every 2-4 inserts works well for them.
Once the shell/cover and insert are put together, a Hybrid / All-in-Two Diaper goes on very easily. There's no folding necessary like you would do with a prefold or flat diaper plus shell/cover combo and there's generally less bulk than with a fitted diaper plus shell/cover combo. Generally, people choose Hybrid / All-in-Two Diapers for daytime use as most will not be absorbent enough for overnights. They are also a great choice for on the go, as packing a diaper bag with a couple of shells/covers and several inserts will take up less room than the same amount of pocket or all-in-one diapers.