What does "Love looks not with thy eyes, but with thy mind, and therefore is winged Cupid painted blind" mean in A Midsummer Night's Dream?

Helena says that love can trick you into seeing beauty where it does not exist.

Helena is upset about the circumstances surrounding her relationship with Demetrius, and the fact that Hermia’s father Egeus wants her to marry him.  Hermia is not interested in Demetrius.  When her father doesn’t relent and Thesues tells her that she has to listen to him, Hermia decides to run away with Lysander.

Hermia makes the mistake of telling Helena about her plans.  They have been friends for a long time, and she thinks she can trust her because she really has no plans for Demetrius.  Helena doesn’t believe her.  In a soliloquy, she says that things would be much easier if she wasn’t in love, because love messes with our heads.

Things base and vile, folding no quantity,
Love can transpose to form and dignity:
Love looks not with the eyes, but with the mind;
And therefore is wing'd Cupid painted blind:
Nor hath Love's mind of any judgement taste;
Wings and no eyes figure unheedy haste:
And therefore is Love said to be a child,
Because in choice he is so oft beguiled. (Act 1, Scene 1)

Cupid supposedly brings love to unsuspecting people by shooting his arrow.  When Helena says that Cupid is blind, it is because our judgement is altered when we are in love.  Love turns everything around and inside out, so we see things that aren’t there or things seem better than they are.  She says that when you are in love, you are easily tricked or fooled, because love makes you childlike.

The concept of loving someone who may not love you back is one continued throughout the play.  When the four lovers go into the forest, they enter the world of the fairies.  They get manipulated by Puck, who has a potion that makes you fall in love with the first person you see when you wake up.  Eventually, the rightful pairs end up together.