The “measure” is the width of a body of type. This little tool helps you find it with a simple click in the toolbar and highlight of a line of text.
WHY WOULD I DO THAT?
The length of a line of type impacts how readable it is. Generally, a good measure is between 45–80 characters per line.
FOR DESIGNERS, FOR DEVELOPERS
Designing legible content requires a lot of attention, across the huge range of device types and screen sizes. A good measure on a desktop monitor might be 80 characters, but on smaller screens or multiple columns might be 45 characters. Although this could all go out the window depending on the colours or typeface used. So do remember to apply your own discretion, too!
Measure makes it easy to be more precise when creating for the web.