'Pat a cake' is a children's nursery rhyme which has accompanying clapping actions in time with singing the song. You can sing it together with a friend and clap your palms against theirs at the appropriate moments.

Being 'good at it' means remembering what claps go together with the song and being able to time and perform it well with your friend.

Pat-a-cake, pat-a-cake, baker's man. Bake me a cake as fast as you can! Pat it and prick it and mark it with B, And bake it in the oven for baby and me!