An encore is an extra piece that is played at the end a performance. It is not on the programme, although the performer may have secretly planned it. A performer will play an encore if the audience claps a lot. Sometimes the audience shout "encore!" when they want an encore. The word "encore" is the French word for "again".
An encore is usually a short piece. Often it is a fast piece to finish the performance off happily, but if a musician has just played a very energetic, loud piece, then the encore might be a quiet piece. A concert given by a solo performer or small group of performers is more likely to finish with an encore than a concert with an orchestra, but a lot depends on the tradition from one country to another.