KPI = Key Performance Indicator = "thing we decided is important for us to measure and increase"

As such exactly what it is, will depend on company/industry/market/team/project/etc. It may be revenue or units sold or widgets made or turnover or customer satisfaction or days without accident etc. In principle, it communicates to teams what is important to business and helps everybody focus and sync.... with usual real-world caveats and implementation risks.

I imagine here, for a large-company linux team, a "KPI" was "how many commits you made" - they measure it, they track it, and reward/recognize/punish team members based on it. So people start working toward increasing the KPI in any feasible way - if you only want me to increase the KPI of commits done, OK, fine, I'll do some commits.