I love this line.
It's the title of one of my favorite books on software usability, written by Steve Krug.
The human-computer interaction pros refer to this principle as "reducing cognitive load." Don't waste precious brain resources on stuff the application should be able to do quickly, accurately, and invisibly.
Here's a prime example.
In displaying lab results, why not do the math for the clinician user? Don't make me calculate how long ago the last lab result was obtained. Tell me it was about 2 weeks ago. The precision can get more relaxed the longer ago the result was obtained.
The same principle applies to displaying the patient's age. Don't just show me the date of birth. Do the math for me.
I have plenty of more important things to think about.