If you are just starting out on the road to selecting your first ukulele, you may see many references to something called a "Low G tuning", but what does that mean?
As I have explained in my tuning sections of the beginners guides, Soprano and Concert ukuleles are most commonly tuned in GCEA tuning (and some tune Tenor ukes this way too). The standard way of tuning GCEA is to have the G on that string that is nearest the ceiling actually a higher G than the next string (the C string). This is called 're-entrant' tuning. If you imagine the keys on a piano keyboard, you will probably know that the notes from A through to C repeat up the keyboard. By tuning with a high G on a ukulele, the G string is not tuned to the G below C, but the G above C. This gives the ukulele its bright uke like sound and is the traditional tuning method. If you have a soprano uke you will note that your G string is thinner than the next string along (the C), not thicker, and this is because it is designed to be tuned higher. Basically as you run up the strings from 1-4, you hear notes that get progressively lower up to the C, then go higher again with the G. With high G or re-entrant, that G string is actually the second highest string on the ukulele behind the A (string 1).
Low G tuning means tuning that G string to the G that is below the C. This is the G that is one whole octave below the high G. Whilst you are still playing the same notes (it's still GCEA) this cuts out the brighter G and makes the ukulele sound a little more mellow or bassier. To do this, I would not however advise using your standard high G strings as that thin G will end up too slack for such a low tuning. To do so, you need to ideally purchase some low G strings which have a thicker G string. In some cases you can get what is called a "wound" G string - this is a normal string wrapped in a thin coil of steel wire much as you will find on an acoustic guitar. A wound string accentuates that low G even more. Equally, many brands offer 'unwound' or plain low G strings. The compensate for this lower note by making the G string much thicker. In fact, in these cases it will be thicker than the C string.
Personally, I would suggest that if you are trying this, you should really only do so on a concert or tenor uke, as the Soprano is really designed for the high G sound, but then you know Got A Ukulele - no rules - if you want a low G soprano, that's up to you! I personally think the low G makes the uke sound too much like a guitar, and prefer the high G sound. In fact many would say that the high G sound IS the sound of the ukulele. That said, I can think of some songs where that extra bass of a low G would really add to the strum.
As a final couple of points, bear in mind that if you are installing a heavy wound low G string or a thick unwound string, you may well need to widen your nut slot for that string, and that isn't something that is easily reversed. Some ukuleles come with a removable nut designed for people who want to switch tunings (i.e. you prepare two nuts and swap them when needed). Also, if you tune your ukulele using a standard pitch pipe as opposed to a clip on tuner, you may struggle using it to tune the G, as it will be made to sound a high G. Also bear in mind that with a heavier string gauge you may also need to adjust the break point at the saddle as the intonation may go off on the G string with this sort of a change. Some people are happy doing this back and forth, but equally a lot of people buy more than one ukulele and keep one in low G tuning!
And that's it really. A low G is still a G, and the ukulele is still tuned in the same key. It's just that 4th string is tuned an octave lower and becomes the lowest tuned note on the instrument. One of the beauties of it is that with chords, as the ukulele is in the same key, you can play along with exactly the same chord sheets as any other C tuned ukulele. Melody lines however will require a bit of adjusting. (Incidentally, you can also go low and high on baritone ukulele tuning, where some people like a high D on the 4th string, but equally, some want a low D. Same point applies.)
Good luck, and give it a try. It can totally change how your ukulele sounds.