Can you explain what the letter I in the SOLID principle means?

The letter "I" in SOLID principles stands for the Interface Segregation Principle.

The essence of this principle is that clients should not depend on interfaces they do not use. It is better to create several specialized interfaces rather than one large universal one.

For example, if you have an interface with methods for reading and writing data, and a class only uses reading, it should not depend on writing methods. It is better to split the interface into two: one for reading and one for writing.

This improves modularity, simplifies maintenance, and testing of the code.