Back when trucks had points you could adjust, dwell angle was what you gapped your points at. The greater the dwell, the longer the points stayed closed,(conducting electricity). On your tester the black lead goes to ground, the red lead went to the tach lead on your coil. First you set your idle, then noted what your dwell was and, (on Fords), turned your truck off, popped the dizzy cap and set your points. It was kind of a trial and error thing untill you got the hang of it. Che@#y's and othe GM's had a nice little door on the cap that used a dwell tool. Looked like a hex head bit on the end of a flexible spring. You could set dwell with the engine running.