You can think of tarot cards as a single piece of a jigsaw puzzle. On its own each card has a meaning, but it is only through a combination with other cards in a structured manner that we are able to derive deeper meanings. A look not just at the card itself, but a study of its position in the layout of the cards and its relationship with the other cards, enables you to see the whole picture.
A tarot reading takes two forms. You can have a reading regarding any specific questions that you may have or you can have an open reading where there's no original question. Such a reading highlights important influences in your life and issues that you have faced during that time.