The simplest thing is the easiest thing to do or understand.
Simplest things are usually good because they are easy and not hard to do or understand.
Examples:
This is the simplest way to do it.
The simplest answer is often the best one.
She chose the simplest dress to wear.
Eating ice cream is the simplest way to be happy.
The simplest things in life are often the most important.
The simplest solution is usually the right one.