Chances are if you've clicked on this page your probably fairly new to ukes and stringed instruments, so a few words to demistify how to use the capo and what they do.
In essence they are a device that clamp the strings down to raise the pitch of the instrument . They are pretty cheap to buy £3-15 or $5-20 and very insticntive to use. Here is a random video of a bloke doing just that.
Their genius with the video uke jam is by moving the pitch by a semitone you can play a whole new set of songs with easy shapes like C chords . Good luck !