I'm no expert, but I do know that the following is greatly simplified:
Izotope Ozone is a popular piece of software for mastering. One reason for it's popularity is that all versions of the software include a large set of presets that can then be tweaked to taste. Used tastefully, Ozone can make the process of evening out levels and raising overall volume without crushing the dynamics easier.
It's also possible to the same thing within a digital audio recording/mixing program (DAW) which these days include a good number of limiting, normalizing, EQ, and so on features. One reason folks buy Ozone is that those Ozone presets let them "in the ballpark" quicker than learning how to use those features that are already included in their DAW.
The folks who make Ozone have a useful not-too-long and not too hard to understand guide to the mastering process that is available for download at
http://downloads.izotope.com/guides/...gWithOzone.pdf
Yes, it uses their product for the examples and therefore is a (reasonably gentle) sales pitch, but it's worth looking at.
I have no connection to the Izotope company. I used to use Soundforge for mastering back in the 20th Century and then tried using the built-in components in a couple of DAWs before eventually using Ozone for most of my mastering because I got results I liked better more easily.
If you are just doing mastering of solo acoustic guitar, Ozone may be overkill. When it's recorded well, you're already nearly there. Just a little normalization and taming of peaks in a DAW may be all you need. I use Logic (MacOS only program) and Adobe Audition (either Mac or Windows, but now only sold on a subscription model). Audacity (free, but fewer features) or Reaper (low cost, more features) could be tried first. Both Audacity and Reaper are available for Windows or MacOS.