Let us remember that a symbol is any object, character or action that stands for both itself, its literal meaning, and also for a larger concept that reaches beyond it. If we look at this very famous poem and profession of love, we can see that it contains a number of different symbols. However, perhaps most clearly we can see that the third stanza contains a symbol of the permanence of the speaker's love for his beloved:
Till a' the seas gang dry, my dear,
and the rocks melt wi' the sun!
And I will luve thee still, my dear,
while the sands of life shall run.
There are a number of examples of this constancy, as the speaker uses the symbol of the seas going dry, the rocks melting in the heat of the sun and the sands of life running. Each of these three actions function as a symbol that stand for both themselves literally but also show the immutability of the speaker's affections for his beloved.