Spiritual Meaning:
A guardian is one who watches over us and spiritually this usually means a guardian angel. This belief in guardian angels can be traced throughout all antiquity and as we become more aware of the existence of the spiritual realms we become able to sense their presence in dreams. As we progress it is natural to meet our own guardian spirits and those who are the guardian spirits of Nature.

Psychological / Emotional Perspective:
In dreams, when we become conscious  of someone standing guard or guarding an object we perhaps need to be careful of our actions. If it seems that we are being guarded, then we are being protected from harm. If we are the guardian then we should consider our responsibilities.

Everyday Material Aspects:
To be a guard in dreams suggests a sense of duty or obligation, either towards someone or to whatever the object is that we are charged with looking after. This also suggests a particular code of conduct or behaviour.

You might also like to consult the entry for Angel.