This is the plug for the Robot Professional Basket, made from food grade Silicone. It is used to block the flow of water from the bottom, making it handy for both pre-heating the piston and verifying the functioning of the pressure gauge
For pre-heating, ensure the basket is clean and free of coffee particles. Push the blank plug down to the bottom of the basket and add hot water as usual, filling it up to the top. Insert the basket into the machine and hold the levers down for 30-60 seconds to warm up the piston. Remove the portafilter, discard the water, and pull out the plug.