Shotgun Surgery smells occur when a single change requires edits in many classes.
The consequence of this is that you can easily cause errors whenever a change is made that outside classes do not expect, limiting how quickly you can make changes to the code.
The aim should be that code changes and classes should have a one-to-one relationship (each change affects only one class). Shotgun Surgery smells are one-to-many (one change affects many classes).
Shotgun Surgery is the opposite of Divergent Change.
Books about code smells
Learn to write better smelling code.
Refactoring: Improving the Design of Existing Code
Code smells: A Predictor of Remarkable Quality Improvement
Clean Code: A Handbook of Agile Software Craftsmanship