Like Bob says, dwell is how long (in degrees) that the points remain closed. The significance is that it relates to the time the coil has to fully form its magnetic field, before being forced to discharge. More dwell gives the coil more time to “charge”. Of course, at low RPM the coil has way more time than it needs. Lower dwell could become a factor at high RPM.
In your case, Mike, I would leave it at 50 degrees. As the points wear the dwell will increase on its own...unless you plan to go racing??