Fingerspelling is making an alphabet with your hands. Sometimes you can also make numbers. Many deaf people learn fingerspelling. They use it when they want to sign a word but there is no sign in the sign language. Other people also use fingerspelling to talk in secret, to help remember something or to talk without making sound (for example in a church).
Fingerspelling is a part of some sign languages. The picture shows the fingerspelling in American Sign Language. Fingerspelling is not the same everywhere. Different countries have different ways of fingerspelling. Some alphabets use one hand, some use two hands.