There are many videos on YouTube explaining audio compression. But this one is quick and includes helpful graphics, in addition to audio samples, to drive home the point.
If you are new to mastering and thought that a compressor was part of a refrigerator (yes, a compressor is part of a fridge but that’s a different type of compressor altogether), this is a good place to start. The vocal example presented is singing, not VO narration, but this is still a good explainer video for VO newbies looking to understand exactly what audio compression is and why it is important and useful.
The video refers to a particular Waves product Renaissance Compressor but the concepts presented apply generally to audio compression using any Compressor plug-in and with any (Digital Audio Workstation (DAW) software.
Transparency Disclaimer: I have not been paid by Waves for this video recommendation. There are no affiliate links on this page. I have no agreement or relationship with Waves for promotion of their products. I have purchased and use some Waves audio FX plug-in tools.