CVSS
Also known as Common Vulnerability Scoring System
A standard scoring system rating vulnerability severity from 0 to 10 based on exploitability and impact.
The base score describes the vulnerability in isolation. Temporal and environmental scores adjust for exploit availability and for how the issue sits in your specific environment, and almost nobody calculates those.
Using base scores alone as a remediation queue is a mistake. A critical in an isolated internal system can matter far less than a medium on your internet-facing login page.
Where this comes up
CVSS sits inside our penetration testing work. If you are trying to work out what this means for your own environment rather than in the abstract, that is the page worth reading next, and a short call will get you a straight answer faster than either.