What is a question mark?
A question mark ? means that the sentence before is a question.
Historically, the ? was formed from a combination of a Q and an O, 'shorthand' for latin Questio. As parchment was expensive, they made a ligature of the tail of the Q and a dot for the O.
A problem with reading texts (especially out loud) is that only at the end of the sentence you will see that it's a question, and your pitch should have gone up at the end (that's how we ask questions). To solve this, some languages (e.g. Spanish) also have an upside-down question mark at the beginning of the sentence, so the reader will know what to expect.
(btw they do the same with exclamation points)