Friction pins are a type of connector peg that connects different Lego components together.
The gray friction pins allows two beams to rotate freely.
The black friction pins provide more friction than the gray ones and do not allow the beams to rotate as easily. The black friction pins also come in a double length size.
 
You can use the black friction pins as rivets in the beams. This is a very strong and reliable way to extend the length of the Lego beams.  
Adding a flat plate on the top and/or bottom gives the extended beam even more strength.