What does it mean when in music the sheet says "Key of..."?

Question:

What does it mean when in music the sheet says "Key of..."?

Like Key of E. What is that?

Best Answer - Chosen by Asker: It tells you three important things:

1. The main note the piece revolves around (called the tonic) is E. This is the home pitch, the resting point.

2. The main chord the piece revolves around is E major. This is the home chord, the resting point.

3. The scale of the piece is E major. While not every pitch you'll hear will come from that scale (outlined above), it's the main focus.