Caravel is the name of the harness provided by Efabless to make it easier to submit a design to the foundry.
Among other things, it provides:
The padring 3kB of memory A small RISCV processor called PicoRV32 GPIOs A logic analyser A Wishbone bus For the full details, check the documentation
The simplest way to interface between your project and the PicoRV32 is probably the logic analyser. This gives you 128 ins and outs that you can use to configure or debug your design.