Level
Sometimes you need a key that doesn’t fit in the staff. The low C is an example of this situation. This note is also called the Central C. You’ll need this note in many songs when you first start playing piano.
2
You see that the note is slightly separated from the staff, and that a line goes through it. We call such a line an auxiliary line. That's actually a very short sixth line near the staff. The middle C is just below the low D.