Launch lugs and rail buttons / guides are how you attach your rocket to the launch pad.
Launch lugs are pieces of tube glued to the outside of your rocket that slides over a launch rod. You will want to design your rocket to use a 6mm rod for UKROC if you go with this option.
Rail buttons come in 3 main sizes: 5mm, 6mm and 8mm. These are attached to your rocket and then slide down the rail. You will want to use 6mm buttons and rail for use with UKROC typically. It's very unlikely you would want to use anything else in this regard.