Shift forks move the synchronizers responsible for changing gears in a manual transmission into mesh with the desired gear. They typically serve two gear ranges. They lock one gear to the shaft when moved forward and the other gear when they're moved backward.
Shift forks are attached to the shift rail, and they slide into a groove in the synchronizer sleeve. The synchronizer spins freely on the fork's two fingers. Most have smooth nylon pads that offer less resistance and noise while in operation.
The groove in the synchronizer is made of a harder material than the fork; forks are typically made of an aluminum alloy. They sometimes need replacement because they wear where they fit into the synchronizer's outer sleeve.