String gauge, thinnest to thickest 1 to 6. Strings come in standard and custom gauge sets. A 12-53 standard set which would be considered light gauge. 11-52 would be a lighter gauge standard set, ergo less tension to fret.

Watch this. It'll bring you up to speed.

You'll also want a string winder and something to snip off the excess string if you're doing it yourself. With a single guitar only changing occasionally a simple $2 hand winder type is fine, and any respectable pair of pliars you might have in the garage will have a cutting edge adequate suitable for snipping surplus string ends at the tuners. If you want to spend more on a string winder, quality, functionality and versatility of this beats anything D'Addario and other high profile brands make at a fraction of their cost.