When it is placed on the second line of the staff like this (notice how it curls around the second line?):



It means any note written on the second line is now the note "G". This is the same note as the third string "G" on your guitar. When you see this note you will play your third string!



Once you know what one note it, by going up and down, you know what all the other notes are. Going up one note from G, as we noted above, goes to the next space, which is an "A":



And down from "G" to the space under it would be "F":