What is depression even? What are the symptoms, how to you know you’re depressed?

I …
…Have almost no motivational drive to do anything anymore.
…Am tired all day
…often feel hopeless and hollow(like I’m dead)
…Feel like I’m just watching the world from outside my body while still being trapped in it
…don’t want to be around people, just want to curl up with several blankets in a small space like under the bed or in my closet.