The measure of a paragraph Is the number of characters (letters, numbers, spaces, punctuation, etc.) wide it is. I.e. Here’s a line of text:
Every morning, I love good type and hate sheep-thieves.
The measure is: 45
If the measure is too short your paragraph will look like a thin column, forcing the eye to trace back far too often. If the measure is too long, the eye’s trip back to the start of the next line will be nearly impossible.
The sweet spot is between 45 and 70.