The CM3+/Lite Compute Module contains the guts of a Raspberry Pi 3 Model B+ (the BCM2837 processor and 1GB RAM). It's the lite version without onboard eMMC Flash.
This is all integrated onto a small (67.6mm × 31mm) board that fits into a standard DDR2 SODIMM connector. The processor interfaces are available to the user via the connector pins. You get the full flexibility of the BCM2837 SoC (which means that many more GPIOs and interfaces are available than with a standard Raspberry Pi), and designing the Module into a custom system should be relatively straightforward because the Pi Foundation has put all the tricky bits onto the Module itself.