According to Price and Folsom, Whitman's encounter with the lilacs in bloom in his mother's yard caused the flowers to become "viscerally bound to the memory of Lincoln's death."

According to Gregory Eiselein: 
Lilacs represent love, spring, life, the earthly realm, rebirth, cyclical time, a Christ figure (and thus consolation, redemption, and spiritual rebirth), a father figure, the cause of grief, and an instrument of sensual consolation.
 The lilacs can represent all of these meanings or none of them.
 They could just be lilacs.
According to Price and Folsom, Whitman's encounter with the lilacs in bloom in his mother's yard caused the flowers to become "viscerally bound to the memory of Lincoln's death."
According to Gregory Eiselein: 
Lilacs represent love, spring, life, the earthly realm, rebirth, cyclical time, a Christ figure (and thus consolation, redemption, and spiritual rebirth), a father figure, the cause of grief, and an instrument of sensual consolation.  The lilacs can represent all of these meanings or none of them.  They could just be lilacs.
When Lilacs Last in the Dooryard Bloom'd