The article mentions designs where the muons can circulate 2000 times, so I think this publication is assuming some conceptual knowledge on the part of the reader.
Muons are generally produced by hitting a target with a beam of something else, so they will not be born at rest. The problem is that making a beam this way is somewhat "splattery" and the resulting components of velocity will not be 100% down the beam path. These perpendicular components need to be removed as much as possible, a task which is referred to as "cooling". Muon cooling is still in its infancy. My friend did his PhD on the project mentioned in this article:
https://www.nature.com/articles/d41586-020-00212-3
Muons are generally produced by hitting a target with a beam of something else, so they will not be born at rest. The problem is that making a beam this way is somewhat "splattery" and the resulting components of velocity will not be 100% down the beam path. These perpendicular components need to be removed as much as possible, a task which is referred to as "cooling". Muon cooling is still in its infancy. My friend did his PhD on the project mentioned in this article: https://www.nature.com/articles/d41586-020-00212-3