What does the upside-down question mark in Spanish mean?

The upside-down question mark is a heads-up that the following sentence or phrase will be a question.

In Spanish, without alternating your tone when you speak or your thinking when you read, declarative (sentences that state something, an observation for example) and interrogative (questions) sentences are indistinguishable from one another. The "upside-down" question mark helps the intended audience understand what is being said enabling them to change the inflection in their voice according to the punctuation.