A "rule of thumb" is an easy-to-remember rule that you can use to make a decision. 

Some useful rules of thumb include:

• Don't date someone that you work with.
• Try to get 8 hours of sleep each night.
• Put at least 10% of your pay into savings.

"Rules of thumb" aren't always true, but they're true most of the time, so you can follow them unless you have some better information to use instead.

The phrase "rule of thumb" is most often used in sentences in two ways:

I don't date people I work with, as a rule of thumb.

A good rule of thumb is to put 10% of your pay into savings.