Shifter bushings are plastic bushings and grommets that provide a tight fit and that smooth feel while keeping the shifter firmly in the proper shift rail. When a shifter bushing is worn, the shifter feels loose. It's because the worn bushing left a gap between the handle and the shift rail.
Shift rails have detent notches machined into their sides. A spring-loaded ball fits into the detent notch to keep the transmission in gear. The transmission will jump out of gear if the ball slips out of its notch. The shift rail should slide easily in its bore. It should not have any lateral up and down or side to side movement. Check each ball and spring for wear and tension; a weak spring can cause the transmission to slip out of gear.