is title is simply to match up the cards in each level into pairs.
In Memory five nights at freddy's you can pick from several difficulty levels. This determines how many cards appear for you to flip over to find their matches. To flip a card over, just tap it.
If you want to put your memory to the test, in Memory five nights at freddy's you can try to recall the location of each card featuring the characters from Five Nights at Freddy's. Put all the matches together and get the max number of points so you can unlock the following levels.