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", but what does that mean?
As I have explained in my tuning sections of the beginners guides, Soprano and Concert ukuleles are best 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). 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, not thicker, and this is because it is designed to be tuned higher.
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 (GCEA) this cuts out the brighter G and makes the ukulele sound a little more mellow. To do this, I would not however advise using your standard high G strings as that thin G will end up too slack for 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.
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.
To do so is purely personal choice, but it's really up to you. I personally think the low G makes the uke sound too much like a guitar, and prefer the high G sound. 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, you may well need to widen your nut slot for that string, and that isn't something that is easily reversed. Mainland 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.