What is "concise"?
To define this, let's first define two other words: succinct and comprehensive. To be succinct is to be short and to the point. To be comprehensive is to fully explain.
To be concise is to be both at once - succinctly comprehensive. It also necessarily implies high quality.
How are things "concise"?
People are concise when they speak clearly and accurately.
Objects are concise when they exclude the unnecessary.
Writing is concise when it presents enough of the important facts in plain language.