There are six holes on the tin whistle. The first three fingers of each hand are used to cover the holes. The little finger of either hand is not used.
Different notes are produced by covering different combinations of these holes as you blow through the instrument. The accompanying photo demonstrates the basic playing position. Notice that the left hand is above the right.
The numbers on the fingers show which fingers should be used to cover the holes as demonstrated in the previous photo. Throughout the book new notes are introduced with photos of the section of the tin whistle which contains the holes used to produce notes.
The one shown opposite demonstrates a B note. This note is produced by covering the top hole with the first finger of your left hand. On the photo this is indicated by a white number inside a red circle covering the hole. The notation for this note is shown.
This B note is written on the middle line of the staff.