What is symbolic

what is symbolic

"Five score years ago, a great American, in whose symbolic shadow we stand today, signed the Emancipation Proclamation. "
I think that you should show the context.
If you check a dictionary, you can see the following explanation.
“If you describe an event, action, or procedure as symbolic, you mean that it represents an important change, although it has little practical effect.” (COBUILD)

(1) We stand today in Lincoln’s symbolic shadow.
(2) Five score years ago, Lincoln signed the Emancipation Proclamation.

King thought that the shadow in which ‘we’ stood then was symbolic. The shadow represented an important change, although it (=the shadow) had little practical effect.

Shadow is an interesting expression. The word “symbolic” was used to modify “shadow”.
“If you live in the shadow of someone or in their shadow, their achievements and abilities are so great that you are not noticed or valued.”(COBUILD)