The piece sign looks like you are slicing a piece of cake, pizza. or pie. Hold out your flat, open non-dominant hand, with the palm facing you. Then take your flat dominant hand, then acting like the knife, have it mimic cutting right down the center of your non-dominant palm.
Teach your toddler the piece sign when you are serving a tranche of food during a family meal or picnic. Teach it as well when you are organizing or laying out your toddler's clothing, toys, or school supplies. As your toddler advances in grasping abstract concepts, they will see that 'piece' refers to a small portion of a larger set or ensemble.
It is good to teach this sign along with other reminders like sharing and waiting one's turn, or organizing one's items so that nothing gets lost.