A tilted uterus is also called a retroverted uterus. It means that it's flipped around backwards. If you google images it should explain that a little better.
Your Ob/Gyn can usually tell you if you have one by pushing hard on your lower abdomen during an internal exam, or sometimes by ultrasound. I found out after I had my miscarriage. It can sometimes cause pain during sex because of the angle and position of the cervix, but usually it doesn't interfere at all with pregnancy. Some studies suggest that it might be more difficult to conceive in missionary position because the angle of the cervix causes it to dip up away from the semen pool instead of down into it.