This is perhaps the sacrament which is most difficult to understand. It used to be the sacrament which gave people the right to receive the Eucharist, but an increasing number of churches now admit children to Communion before confirmation. In the ancient Church people were admitted to communion after baptism.
Some people think of confirmation as a sort of "topping up" with the Holy Spirit, but this makes little sense if we believe that the gift of the Spirit is fully given at baptism.
It is perhaps best to think in terms of confirmation as our opportunity to confirm that we are prepared to keep the promises made by our parents and godparents at our baptism.