An upper storey.
"The children played upstairs while their parents cooked in the kitchen below."
Located on a higher floor or level of a building.
"The children played upstairs while their parents worked in the kitchen below."
In plain English: Upstairs describes something located on a floor above where you are standing, usually inside a building with more than one level.
"The upstairs bedrooms are always quiet during the day."
Usage: Use "upstairs" as an adjective before a noun without adding "the," but place it after the definite article when modifying a specific location, such as in "the upstairs bathroom." Avoid using this term to describe objects that are physically elevated within a single room unless they are situated on a raised platform or loft.
Up the stairs; on a higher floor or level.
"After dropping my keys, I had to run upstairs to find them in the living room."
In plain English: Upstairs means being on the floor above where you are standing now.
"I left my keys upstairs in the bedroom."
The word upstairs comes from combining the prefix up- with stair and adding the plural suffix -s. It literally means located on or above the stairs.