The adjective bittersweet describes a taste that is a blend of bitter and sweet. Bittersweet chocolate, for example, contains more chocolate and less sugar than milk chocolate or even semisweet chocolate.
The adjective bittersweet doesn't just refer to taste. It can also describe a blend of emotions that are sweet but also tinged with sadness. If you attend a memorial service for your grandmother, for example, you can enjoy telling stories of the things you did together, but still feel sad that she's gone. The feelings and memories you have make you simultaneously happy and sad, and are therefore bittersweet.