Ukulele players use a capo to raise the pitch so they can play in a different key using the same fingerings as playing open (i.e., without a capo). In effect, a capo uses a fret of an instrument to create a new nut at a higher note than the instrument's actual nut.

Here are a couple songs that you need a capo to play in key with the original:

For $5, I'll bring you a capo for your ukulele.

Buy a Capo