it's a punctation mark used to add on to a sentence. The sentence is continued, rather than ended. People suffering from depression and suicidal thoughts see it as a symbol of strength. It gives them the courage to keep living and to not "end their sentence," (life).
Well sometimes it can mean if someone was depressed that now that part of their life is done but their life is also not yet over. Because that's what a semicolon does; it could be at the end of a sentence but the sentence continues, as does that person's life.