Gold surrounded the figures of Christ, Mary and/or the saints: it is a symbol of light (already during byzantine times) and thus of divinity.
The background was produced by applying thin gold sheets often enriched by stamping.
Sacred works which are called "gold background" are mostly paintings produced for churches and private customers between the 13th and 15th centuries.
The procedure was very expensive due to the kind of raw material needed. It shows however the willingness to be generous to honour God in the best way.