The first thing we need to establish to answer this questions what era Athol Fugard's masterpiece is referring to. It is Apartheid South Africa, in which black people were maligned by a number of draconian laws which were officially passed into law in 1948.
This play is set in the 1980s, just before these laws began to gradually be repealed. Our main character, known as Mr. M, is attempting to create cohesion between White teens and Black teens by organizing a a debate between seniors at Zolile High School (a school for Black students) and Camdeboo Girls' High School (a school for White students). The message that Mr. M is getting across here is that segregation is silly, and that education should not be concerned with skin color.
Later, Mr. M winds up being killed in a violent township protest after being accused of being an informer. The message Fugard seems to be conveying here is the violence of Apartheid and the risks that anyone who thought differently to the government faced.
Thami and Isabel, students who were involved in the debate, both grieve terribly in the aftermath of Mr. M's death. In the aftermath of his passing, they realized the value of the message that he had been conveying to them—that the fight for a world not segregated by color could begin with something as simple as an inter-school debate.
I would argue that anyone who has ever suffered any kind of prejudicial treatment would be able to relate to this great play.