What does the Belarus flag represent?

Technically there is no official interpretation of what the color represents, but Lukashenka (the leader/dictator of the country) has said that "red stands for the freedom and sacrifice for the nation's forefathers and that green stands for life."
The decorative pattern on the side of the flag is one of the kinds of traditional patterns commonly used for a traditional 'rushnik'*. The pattern is derived from local plants and flowers. 

*a woven towel used for ceremonial events like religious services, funerals, and other social functions.