Distance between the strings and the fretboard. In the 12th fret, the distance between string and fret should be about 2.5 to 3.0 mm. Higher strings reduce unwanted buzz, but require more pressure when single-note picking. Too low action can cause a loss of volume in addition to buzzing. The correct distance may vary depending on the thickness of the strings used (a C string should therefore be higher than an A string).

The position of the strings is controlled by the height of the nut and bridge. If the action is too high, the saddle can be filed down and the notches of the nut can be deepened.

There are special string action gauges available for adjusting string heights and string spacing.

Pictures and videos

String action gauge in cashcard format

String action is tested at the 12th fret
(From: 6 Things That Make a Ukulele Play Great)

Sponge Bob Ukulele Setup I

Sponge Bob Ukulele Setup II