A resin joint is a mould in which you make the cable connection and then fill it with casting resin. The mould holds the liquid resin in place until it has cured, allowing you to tidy your cables away neatly and bury the connection safely.
Let the filled cable box cure for an hour, screw on the lid and bury the box fully covered.
Please note: check that the connection works before pouring in the resin. Once the resin is in, you can no longer access it.