what does the red on the german flag represent
Red and white were also colours of the Hanseatic League (13th–17th century). Hanseatic trading ships were identifiable by their red-white pennants, and most Hanseatic cities adopted red and white as their city colours (see Hanseatic flags)
Principality of Reuss-Greiz
Taking the colours of the banner of the Holy Roman Emperor, the flag of the Austrian Empire was black and gold. Francis II was the last Holy Roman Emperor, with Napoleon forcing the empire's dissolution in 1806. After this point, these colours continued to be used as the flag of Austria until 1918.
The colours red and white were also significant during this period. When the Holy Roman Empire took part in the Crusades, a war flag was flown alongside the black-gold imperial banner. This flag, known as the "Saint George Flag", was a white cross on a red background: the reverse of the St George's Cross used as the flag of England, and similar to the flag of Denmark. Red and white were also colours of the Hanseatic League (13th–17th century). Hanseatic trading ships were identifiable by their red-white pennants, and most Hanseatic cities adopted red and white as their city colours (see Hanseatic flags). Red and white still feature as the colours of many former Hanseatic 
UNANSWERABLE UNANSWERABLE There are many theories in circulation regarding the colour scheme used in the flag. One such theory suggests that the colours were those of the Jena Students’ League.
Flag of Germany
Origins
There are many theories in circulation regarding the origins of the colour scheme used in the 1848 flag. It has been proposed that the colours were those of the Jena Students' League (), one of the radically minded banned by in the Carlsbad Decrees; the colours are mentioned in their canonical order in the seventh verse of August Daniel von Binzer's student song ("On the Dissolution of the Jena Students' League") quoted by in his "Academic Festival Overture". Another claim goes back to the uniforms (mainly black with red facings and gold buttons) of the Lützow Free Corps, comprising mostly university students and formed during the struggle against the occupying forces of Napoleon. Whatever the true explanation, these colours soon came to be regarded as the national colours of Germany during this brief period, and especially after their reintroduction during the Weimar period, they have become synonymous with liberalism in general. 
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Flag of Germany [SEP] Principality of Reuss-Greiz
When Heinrich XI, Prince Reuss of Greiz was appointed by Joseph II, Holy Roman Emperor to rule the then-new Principality of Reuss-Greiz on 12 May 1778, the flag adopted by the "Fürstentum Reuß-Greiz" was the first-ever appearance of the black-red-gold tricolour in its modern arrangement in any sovereign state within what today comprises Germany – the Reuss elder line that ruled the principality used a flag whose proportions were close to a "nearly square"-shape 4:5 hoist/fly ratio, instead of the modern German flag's 3:5 figure. In 1804, Napoleon Bonaparte declared the First French Empire. In response to this, Holy Roman Emperor Francis II of the Habsburg dynasty declared his personal domain to be the Austrian Empire and became Francis I of Austria.