It’s a small, flexible plastic rod. Some implants require one rod, others require two.
A healthcare professional will insert it under the skin of your upper arm (it can go in either arm). They will apply a local anaesthetic (a medication that numbs the skin) to your arm, then use a device that feels like an injection to put the implant in.
It will take a few minutes and most people don’t find it painful. You won’t need any stitches after.