Staves are the foundation of music notation, they are a set of 5 horizontal lines and 4 spaces. Each gap or line is a different note that is decided by the clef. On the keyboard in the key of C, every line or space represents a white note. A stave has no meaning until a clef (Treble, Bass, Alto, Tenor etc.) is placed onto it (see clef section).
Here is an example of a stave being used in a piece of music. This is an extract from the trumpet 1 part in "In The Stone" by "Earth Wind and Fire" (the extract starts 8 seconds into the video). As you can see there are lots of notes across the stave. The stave means you can read across the music from left to right and therefore follow it.
The first note is an F# as it is on the top line. We know this because there is a treble clef at the beginning (see the next section for details) and we know it is a sharp because it is on the first line, because there is a sharp on the same line at the very begging of the stave in the key signature (see key signature section).
Have a go yourself! How many notes are on the second space down?
Staves are very useful to know as they are crucial and are the foundation in music notation so without them you would not be able to write any music. If the stave didn't exist, music would just be a bunch of blobs on a page which would be unplayable.