Stands for Bring Back The Love. It is a reference to people being hostile and unfriendly. Was commonly used by 'hippies' in the late sixties period, aimed at politicians and others whom they felt needed to stop hating everyone, and start loving them. Was used as a statement ie. on protest banners, or stuck in windows, wasnt commonly used in speech.
Person 1: Ahh, whats with all the hate these days?
Person 2: I know right. BBTL all the way.