Coincident angles are angles that have the same terminal side. Another way to describe it is angles that end in the same place. Another way to describe it is keep subtracting 360 (or 2pi) from the larger angle until the result is under 360 (or 2pi). If that result is the same as the smaller angle (the one already under 360 or 2pi), then the angles coincide.
A 45 and 405: 405-360 = 45. The angles coincide.
B pi/6 and 13pi/6: 13pi/6 - 2pi = 13pi/6 - 12pi/6 = pi/6. The angles coincide.