Personal Record (running)

A personal record or personal best (frequently abbreviated to PR or PB) is an individual's best performance in a given sporting discipline. It is most commonly found in athletic sports, such as track and field, other forms of running, swimming and weightlifting.

The term "PR" came from the world of running, referring to your best time in a race of a specific distance. So, if you run a 5K race in 28:45, that's your PR for the 5K distance. If you run faster than 28:45 in a subsequent 5K race, then you have a new PR for that distance..

Over time, "PR" came to mean any new record in a sport that could be performed by a single person. Crossfit and powerlifting have popularized the term because a person's PR is used to measure their current level for a given exercise.

...
Wikipedia