LEGO Technic part Numbers
Beams, axles, panels, and axles connectors usually have numbers next to them, to uniquely identifies the piece:


for beams and axles, the number is the length (length in number of holes)
for panels, it identifies the panel (and the panel mold contains the same number)
for axle connectors, it identifies the angle (again, the same number is part of the mold, and visible on the piece)


Same pieces with different colors have the same number attached