Significance of touching forheads in Tibetan Buddhism (Karma Kagyu)?

I have visited a local group of Karma Kagyu practitioners once, hearing a Dharma talk and participating in the mantra, rituals, and meditation. At this occasion, the members of the Sangha greeted one another by touching foreheads with each other. I didn't ask why at the time, and haven't been able to find a description of that tradition online. Could someone shed some light on this custom?