Complex sentence: Colloquially it refers to British citizens or residents whose parents are of two or more different races or ethnic backgrounds, however it is often used synonymously for people of Mulatto ancestry as this is the largest ethnic mix and often most visibly identifiable of beiong of mixed descent.
Split into simpler sentences: Colloquially it refers to British citizens or residents whose parents are of two or more different races or ethnic backgrounds. In practice however the term "mixed-race" is often used synonymously for people of Mulatto ancestry as this is the largest ethnic mix and often most visibly identifiable as being of mixed descent.