A configuration file stores settings that tell a program how to run. It might say which language to use, where to connect, or what options are turned on.
Imagine a small instruction sheet kept beside a machine. Instead of changing the machine's insides, you update the sheet to choose how it should behave.
Configuration files often use a format with clear names and values so both people and programs can read them. Be careful not to put secret passwords or API keys in a file that may be shared publicly.