If you do or say something tactlessly, you're being rude — or at the very least, incredibly insensitive. Tactlessly tell your host that dinner was "so-so" and you probably won't be invited back.
Acting tactlessly literally means "without tact," and tact is that graceful balance between common sense and thoughtfulness. Don't tactlessly ask your neighbor how her day was when you know her beloved cat died that morning. And if you want to borrow a book from your brother, ask politely instead of tactlessly saying, "You're too lazy to read it anyway!"