6/40 as a percentage is 15.00%. This means that the fraction represents fifteen hundredths of a whole when expressed in percent form.
The fraction 6/40 can be simplified to 3/20, which still gives the same value. It is a small part of a whole, but it is easy to understand when you see it as a percent.
In real life you might see 15.00% on a discount tag, a test score, or a chart showing a portion of a group. Knowing the percent helps you compare it quickly with other numbers.