LRP stands for “long relief pitcher” and usually refers to a non-starting pitcher who is used for more than two innings at a time.
Through most of baseball history, teams have used LRPs on a sporadic basis, calling on them only when the need arises. That typically happens when a starter is chased early in a game. In that case, a manager may bring in a reliever to work multiple innings to try and keep the game in hand so he can transition to the backend of his bullpen — the setup relievers and closer.
In the 2020s, the LRP became more commonplace as teams like the Tampa Bay Rays began experimenting with “openers” and so-called “bullpen games,” which are designed from the beginning to feature several pitchers who each hurl multiple innings.
But there have been some relievers, even in earlier times, who regularly worked several innings per appearance. Some of these guys even served as their team’s closers while racking up ponderous innings totals.
Probably the best example of a long relief pitcher who also functioned as a closer was Mike Marshall, who threw a record 208.1 innings over 83 appearances for the 1974 Los Angeles Dodgers. He also led the team with 21 saves, a combined performance that netted him the National League Cy Young Award.