T20 World Cup: What is Net Run Rate and how is it calculated?

South Africa won 4 out of their five league matches and yet failed to qualify for the semis in the ongoing T20 World Cup on Net run rate. Cruel, right? But that's what cricket is. A game of fine margins.

source https://timesofindia.indiatimes.com/sports/cricket/icc-mens-t20-world-cup/t20-world-cup-what-is-net-run-rate-and-how-is-it-calculated/articleshow/87563113.cms