A runner is slang for a person who goes beyond safe zones for various reasons such as obtaining supplies, setting up traps, scouting, destroying volatile nests, saving other survivors, or even killing other humans.
Generally a runner avoids confrontation with infected and other humans alike. Many runners are skilled in parkour and take advantage of high places. This is probably due to them being skilled athletes most likely being apart of the Olympic-like games that were happening during the outbreak as well as Brecken (referenced in Dialogue with Crane) teaching others after the initial outbreak.
After completing the tutorial you are given fresh clothes, which is a standard runner's outfit. Generally consisting of a light shirt, running boots as well as knee pads. This is not to be confused with the Runner outfit, which is traditional running attire, commonly worn pre-outbreak.