What you see above is the most basic element of music notation. A set of horizontal lines called a staff (plural is staves). The most common staff has five lines.
When we want to place a pitch on our staff, we place an oval called a notehead (shown in red) directly onto a line, or into the space between the lines. As you can see in the graphic above, the note on the left has a staff line directly through its center, so we say this note has been placed on a line. The note on the right occupies the space between two lines, so we say this note is on a space.
Notes that are placed lower on the staff will be lower-sounding pitches, notes that are placed higher on the staff will be higher-sounding pitches.
Each time we ascend on the staff from a line to the next space, or a space to the next line, we move forward one letter in the musical alphabet. We reverse the alphabet when the notes move downward in the same fashion. But how do we know which letter to assign to each line and space? That is where clefs come in...
The symbol you see placed on the left side of the staff above is called a clef. There are various types of clefs, but the clef you see here is referred to as the treble clef or the G-clef. Does it look at all like the letter "G" to you? The treble clef is, in fact, a decorative letter "G" that assigns that letter to a specific line of the staff (the second line from the bottom in red). Any note placed on that line (the note is also in red) becomes the pitch G. Once we know the location of one letter on our staff, we can find all the others:
Reminder: there is no "H" in the musical alphabet, so once we arrive at the letter "G" we start again at "A".