The Mirror of Erised was the obstacle that essentially made all the previous obstacles protecting the Philosopher's Stone obsolete.
Dumbledore had enchanted the mirror so that only a person who wanted to find the stone, but not use it, would be able to get it - thus making it impossible for Quirrell/Voldemort to ever get the stone.
However, we know that Harry stumbles upon the mirror sitting in a random classroom when he is wandering around at night under his invisibility cloak. Of course, there is a theory that Harry found the mirror because Dumbledore was intending for Harry to find it as part of a supposed plan to let him "test his strength" and face Voldemort himself.
This theory has its inconsistencies: firstly, how could Dumbledore have predicted that Harry would run into the mirror? He knew Harry was intuitive and he knew he would be wandering around at night with the cloak, but Harry could just as easily not have found it. I think that if Dumbledore had really wanted Harry to know about the mirror, he would have personally, deliberately gone and shown it to him.
We also know that at this point in the year, Quirrell still does not know how to get past Fluffy, so the stone is somewhere in the third floor corridor past Fluffy, it is not in any legitimate danger, and Dumbledore surely knows this too.
This leaves me to believe that Fluffy and the other obstacles are a decoy and the stone is in fact sitting in the mirror in this random classroom. At first this appears to make much sense, but then why does Dumbledore proceed to move the mirror to the third floor corridor a little while later? Does he do that purely to prevent Harry from finding it again?
Dumbledore clearly does not think that Harry would be able to get the stone from the mirror - but of course he does, and in doing so puts the stone in genuine danger for the first and only time. If Dumbledore has a hunch that Harry might go after the stone, why does he risk everything by actually putting the mirror and the stone in the third floor corridor as opposed to moving it to another hiding place that neither Harry nor Quirrell/Voldemort would ever suspect?