If you mean flight protectors, they are small metal or plastic pieces which are place on the top end of the flight, it is designed to keep the flights in good condition.
The main ones are 501, you start on any number and get down to 0 finishing on a double. 301, same as 501 but it is double in, double out. There is also cricket, you go round the board 1,2,3,4 etc all...