Important is a term used to describe something of great significance. Anything can be important, from physical objects, to concepts or information. If something in a story is important, chances are it's gonna show up or be mentioned a lot. Chances are also that it's a major plot point. Examples include the One Ring, the Force, and the concept of phoenix tears healing wounds.
Some people have trouble discerning what is truly important from what is not. For instance, suethors will often decide that getting their self-insert in bed with their lust object is more important than good spelling, grammar, and characterization.