There are a number of problems that can occur while drilling a well. Whether a drill-string breaks and falls to the bottom of the wellbore or a bit breaks, accidents happen. Even pipe or a tool can fall from the rig floor into the bottom of the well.
This stray equipment that has fallen into the well is referred to as fish or junk, and regular drill bits cannot drill through it. Should a fish fall into a well, fishing is required to remove it.

The application of tools, equipment and techniques for the removal of junk, debris or fish from a wellbore. The key elements of a fishing operation include an understanding of the dimensions and nature of the fish to be removed, the wellbore conditions, the tools and techniques employed and the process by which the recovered fish will be handled at surface