This form of greeting is only performed by men of a certain ethnic group. It is also forbidden for women to greet this way.
Here is the rundown:
Step 1: See someone you know.
Step 2: Lean forward and press one side of your forehead to the opposite side of your buddy's forehead.
Step 3: Alternate to the other side of the forehead.
Step 4: Alternate yet once more
Ironically, for a relatively homophobic country, men here are very comfortable with the man-on-man contact. It is not unusual to see Congolese men holding hands while they walk or cross the street.
In addition to the head-bump, another standard greeting here is the triple cheek kiss. Most people follow this convention, but with such a diverse international community in Kinshasa, you never know for sure what you are going to get.