As a matter of fact they don't represent anything, the colors are taken from the first Belgian flag made in 1790(!) when Belgium became for the first time independent. Then the colours were taken from the crest of the Duchy of Brabant. Where it all started.
The flag was adapted a bit and adopted in 1830.
During WWI this poem was made giving the colours a few meanings
THE BELGIAN FLAGRed for the blood of soldiers,- Black, yellow and red -Black for the tears of mothers,- Black, yellow and red -And yellow for the light and flameOf the fields where the blood is shed!To the glorious flag, my children,Hark! the call your country gives,To the flag in serried order!He who dies for Belgium lives!
Red for the purple of heroes,- Black, yellow and red -Black for the veils of widows,- Black, yellow and red -Yellow for the shining crownOf the victors who have bled!To the flag, the flag, my children,Hearken to your country's cry!Never has it shone so splendid,Never has if flown so high!
Red for the flames in fury,- Black, yellow and red -Black for the mourning ashes,- Black, yellow and red -And yellow of gold, as we proudly hailThe spirits of the dead!To the flag, my sons!Your country with her blessing "Forward" cries!Has it shrunken? No, when smallest,Larger, statelier it flies!Is it tattered? No, 'tis stoutest!