A Hit and Run (HnR or H&R) is when a torrent is downloaded but not seeded for enough time as per the trackers rules. Typically a torrent must be seeded until a 1:1 ratio or a specific amount of time anywhere from 48 hours to 1 week in a 2-4 week period. If a user fails to meet one of those requirements then a Hit and Run is added to the users account. It's important to note that freeleech torrents are not exempt from hit and run rules.
The Hit and Run can be removed by seeding the client until the seed length has been met. If too many Hit and Runs build up on an account then the user may lose downloading privileges or have their account disabled. If a hit and run is earned on a file that the user no longer has access to, they can still redownload the torrent from the tracker or try downloading the exact same file off another tracker so they can start seeding it again, however download stats will still be recorded for the second download.