It's referencing a specific location - where depends on context. For instance, someone working in a supermarket would likely mean aisle number one, whereas in an office it probably means that room number. The speaker is saying that they are going to this location, and if you need them you will find them there.

It means "in number one position; the first". https://twitter.com/LizEkillam71/status/1204103898459332608 I'll be the first to help. Okay, I see we lack the context :-(

in a new room at any Hotel you can take number one number two two number three number four as labelled

This must mean a room that is labeled “1”. They are in (room) number one. So referring to a place that the person is.