Spiritual meaning

Spiritual crusades and journeys often require certain standards of
behavior and flags in dreams will often symbolize an idea or principle.

Psychological meanings

Any national flag will signify a degree of patriotism that may
be necessary, or possibly the need to be more militant. Now with
its connection to international sport, a flag can suggest an opportunity
for exhibitionism.

Common meanings

A flag in a dream will have the same meaning as a banner – that is a
standard or a place round which people with common aims and beliefs can
gather. It may represent old-fashioned principles and beliefs.

You might like to consult the entry for Banner for further clarification.