Upstairs refers to a part of a building that's higher than the ground floor.
When you need something or want some peace and quiet, you can go upstairs. Maybe your room is upstairs, or maybe there's a cozy study or a guest bedroom up there. Some people have an office or a home gym upstairs. If you're visiting someone's house, they might say "My sister's room is upstairs, would you like to see it?" Or, if you're in a hotel, the rooms with beds usually are upstairs, but sometimes there are even floors higher, like executive suites.
An upper storey.
Located on a higher floor or level of a building.
Example: They can sleep in the upstairs bedroom.
Up the stairs; on a higher floor or level.
Example: I hate the people who live upstairs, and I especially hate their piano.
