You are thoughtful if you consider how your actions and words will affect other people’s feelings. It is thoughtful to cook your friend's favorite dish when you invite him over for dinner.
The adjective thoughtful describes someone who pays attention or considers others' feelings when they speak and act. If your friend is heartbroken about a recent break-up, holding off on gushing about your new love is the thoughtful thing to do. Thoughtful can also describe intellectual depth, like a thoughtful essay on a well-known poem that focuses on a line that other scholars have overlooked.