String gauge is how thick your strings are.... make SURE you check what string gauge you put in before you change the strings.
If you don't know what your original string gauge is, I assume that it's the standard .12 gauge.....
The .12 is actually is in inches. Actually... it's .012.... So here are the most common string gauge (but there are so many variations for it), from thinnest to the thickest.

.012 .016 .024 .032 .044 .054

Don't forget..... this is for acoustics!!!! Electric guitars use different gauges.

So when you change your strings, make sure you know what gauge their are in.