Something inessential isn't terribly important or necessary. While you need certain nutrients to stay alive and healthy, things like doughnuts and milkshakes are inessential parts of your diet.
The ornamental fireplace in your house that's never used is inessential, but the wood stove that heats your kitchen all winter is not. Inessential details can complicate books, essays, or legal documents, making them harder to understand. The adjective inessential adds the prefix in-, or "not," to essential, "extremely important," from the Latin root essentia, "being or essence."