"A Lecture Upon the Shadow" is a poem about love and, somewhat metaphorically, about vision. Using an extended analogy (a conceit), Donne's poem compares the vision of lovers (and the delusions of lovers) to a situation of light and shadow wherein sight is only unhindered at noon, when the light of the sun produces no shadow.
The meaning of the poem seems to be that when love reaches maturity it allows lovers to see one another clearly, without illusion (and so free from wishful thinking, from erroneous dreaming and fill-in-the-blanks romanticizing) that characterizes immature love. When the maturity of love fades (or the strength of love fades), the lovers' vision again becomes compromised.
Only when love is at its pinnacle does it produce clarity of sight.
But, now the sun is just above our head,
We do those shadows tread,
And to brave clearness all things are reduc'd.
True love then means that lovers are both willing and capable to love the other person for who they are and to leave off any creative tricks of vision--to see the beloved as he or she truly is.
Love takes on the functional role of providing one with the power to see clearly, although this function exists in a somewhat circular context. Love provides the courage to see without illusion but the lack of illusion is also the definitive characteristic of true, honest love.
When love wanes to a point of lesser power (anything less than full strength), it ceases to provide the courage of honesty and ceases to clarify.
But oh, love's day is short, if love decay.
Love is a growing, or full constant light,
And his first minute, after noon, is night.