Frets, the metal strips across your fingerboard, create different contact or end points for strings, which changes the pitch. (Pitch refers to a specific note/frequency and how high or low it sounds).
When an open string is pressed to the fingerboard just behind a fret, the string touches the fret instead of the nut, and the length of the string that can vibrate changes. Experiment with frets and how they change the pitch of a string by pressing your left index finger in different frets while picking the string with your right index finger. Listen. Does a shorter string length sound higher or lower?
Left hand fingers are numbered starting with the index finger as #1. Frets are numbered according to their relationship to the nut, with #1 being closest to the nut. As you begin to learn to play ukulele, your finger numbers should match fret numbers as in the picture above.
Practice pressing the first three notes on the first string (A with A#, B, C). Keep your left hand and fingers in their correct positions. Then pick the string with your right hand while pressing fretted notes. Now, use the fret numbers in the "Batman Theme" box to play your first melody.