What does the upside down question mark mean?

In Spanish, a question often uses the same words for a question as it does for a statement, so the beginning of a question needs to be denoted with an inverted question mark.

A question like, "Does your Grandmother have cookies?" in English, would translate from Spanish to English as, "Your Grandmother has cookies?".

The person needs an inverted question mark to start the question, otherwise they may begin reading it as a statement "Your grandmother has cookies.", then backtrack when they realize it's a question.