Before the electronic ignition, automobiles had a mechanical distributor. Inside is a rotating cam which opens and closes two contact points. When the points open, that is when the spark occurs.
It is important for the spark to occur at the right moment of engine rotation. A mechanic loosens a bolt so he can turn the entire distributor to the right or left, to adjust the spark timing.
When replacing points, they must be positioned so the cam closes them for the proper length of time (dwell angle). The adjustment is tricky. A small distance can change dwell angle by several degrees. Fortunately you'll have satisfactory operation even if you get the dwell slightly below spec. That is if the coil, condenser, etc., are all in good shape.
(I still have my dwell meter although I have not used it in decades.)
Furthermore when you change the dwell angle, it affects spark timing. Timing is much more critical than dwell angle. So a timing adjustment is necessary as the final step.