If you are tactful, you have a knack for saying the right thing at the right time. A tactful person is appropriate and sensitive, never rude or careless.
Tactful means "full of tact." What's tact? It's the gift for saying the right thing because you understand what the situation calls for. So, if you are tactful, you wouldn't tell your friend that the food at her party was awful — you'd talk about the interesting conversation you had and the great music. But even when tactful people have to give criticism, they do it in such a way that the other person doesn't get offended.