A Version Control System (VCS) allows to organize changes to file structures in unique revisions (aka commits). Each revision stores the change to the file and additional meta information like the author, the date and optional documentation (comments).
Working with Version Control makes it possible to reference specific revisions. With revision being a consistent snapshots of the code base at a specific time, it becomes possible to reason about introduction of failures or start of effects. Without that such discussions are often not even possible.
The nature of especially distributed VCS supplements collaborative software development in various workflows.