In music, a staff is like a canvas for “painting” notes. It’s simple: It’s just 5 horizontal lines. They extend across a page (or screen), from left to right, and notes are placed on them from left to right when someone writes music, and the notes are read from left to right when someone plays music.
[Yes, there are staves (plural of “staff”) with different numbers of lines, but they are rare and have very little, if anything, to do with piano music. So for now — and most of the time, you can just think of 5 lines.]