Clutter is a noun, and it refers to "a number of things scattered in disorder." Clutter could be a pile of papers, or many things in a drawer or closet. The idea is that the things are disorganized, and sometimes unnecessary.
Some people like clutter. They have a pile of papers on the desk and drawers full of envelopes, pencils, business cards, etc. Some people feel more comfortable with clutter in the kitchen, living room or bedroom.
Other people don't like clutter and prefer to keep surfaces clean and minimal, and closet and drawers organized.
Which style do you prefer?
The adjective for this word is cluttered, and the opposite adjective is uncluttered. For example: Although Mary's desk was uncluttered, she wasn't as productive as Joe, whose desk was always full of papers.
Sometimes people use clutter as a verb, and declutter as its opposite. You might enjoy reading a blog post about decluttering on zenhabits.