Around 3/4 in the movie Straight Outta Compton during the riots, two people are holding two scarves, one red and one blue, knit together and walking towards the police.
What's the meaning of this and/or what does it symbolise?
That scene depicts the LA Riots (related to the 4 police officers involved with the beating of Rodney King being found not guilty). The day before the riots broke out, the Bloods and the Crips declared a truce (called the Watts Truce). Many gang members would wear colors (often in the form of bandannas) to show their affiliation (Red for Bloods, Blue for Crips) so most likely the tied red and blue bandannas are to illustrate the truce between the gangs.