Shakespeare writes about love a lot. This quote is about that theme, but what I like about this quote is that it doesn't support the "love at first sight" notion. In fact, the heroic couplet takes sight completely out of the love equation. Shakespeare is telling his audience, through Helena, that love is a matter of the heart. It is more dependent on an emotional bond rather than a physical attraction. The idea is nothing new to Shakespeare. He writes about the same thing in a couple of his sonnets. Sonnet 130 explains how his mistress is definitely not attractive, but he still loves her deeply.
And yet, by heaven, I think my love as rare
As any she belied with false compare.
Sonnet 141 is along the same lines; however, it specifically mentions the eyes just like Helena does in the play.
In faith, I do not love thee with mine eyes,
For they in thee a thousand errors note;
But 'tis my heart that loves what they despise,
Who in despite of view is pleased to dote....
As a general rule of love, I feel that the quote is teaching an important lesson. In terms of the play, the line foreshadows events to come. Most notably, the line foreshadows Titania falling in love with Bottom despite the fact that he has a donkey's head.
Another example of how this quote shows true love being blind can be found through the character of Demetrius. When the play begins, he's in love with Hermia, and Helena indicates that the main reason is because Hermia is prettier. By the end of the play, Demetrius has found his true love in Helena. He was looking for love with his eyes; however, he wasn't finding true love. As soon as he stopped relying on his eyes, he was able to discover the love that he and Helena share.