A slow steady boil, which can also be called a gentle boil, is where the ingredients are being heated slowly enough that the contents are just at the boiling point ... a few gentle bubbles breaking the surface and if the pot is stirred the boiling is interrupted for a few seconds before it resumes. The contents at the surface are cooler and being mixed with the warmer contents at the bottom of the pot which reduces the temp of the total mass to below the boiling point.
In a full rolling boil, or a hard boil, the contents are being heated at such a rapid pace that there many larger bubbles on the surface and stirring will not dissapate the boiling. The contents at the surface of the pot are so close to the temperature at the bottom of the pot that stirring does not alter the temperature of the entire mass enough to lower it to below the boiling point.