Soprano and concert sized ukes are generally tuned to
one of four different tunings. The oldest, and most traditional is
A D F# B (My Dog Has Fleas) - Commonly called "D tuning".

The more modern tuning is the same thing, down a whole step to
G C E A (My Dog Has Fleas) - Commonly called "C tuning".

In both tunings, the 4th string
(closest to the head of a right handed player)
is tuned higher than the 3rd string.
That's what seems to give the uke it's characteristic
"Uke sound".

These days, some players like to tune that 4th string
down an octave. That way the uke is tuned like a
guitar capo'd at the 5th fret (C tuning) or the 7th fret (D tuning).
This type of tuning is commonly called "Low C" or "Low D" tuning.

To achieve the low tuning, we can't simply tune the standard
string down an octave. The tension will be too low.
We have to substitute a larger diameter string for the 4th
in order to achieve the correct tension for low tuning.

The SOPRANO/CONCERT size uke strings listed here
Come WITH an extra 4th string. That string is
a plain nylon, .040 classical guitar string.
That allows you to tune your uke in any of the four popular tunings.