In short, the Single Responsibility Principle (SRP) means each code unit should have exactly one responsibility. This means writing functions that do one thing only, creating types that are responsible for a single thing, and creating higher-level components that are focused on one aspect only.
This means that if your class manages some type of resources, such as file handles, it should do only that, leaving parsing them, for example, to another type.
Often, if you see a function with "And" in its name, it's violating the SRP and should be refactored. Another sign is when a function has comments indicating what each section of the function (sic!) does. Each such section would probably be better off as a distinct function.
A related topic is the principle of least knowledge. In its essence, it says that no object should know no more than necessary about other objects, so it doesn't depend on any of their internals, for example. Applying it leads to more maintainable code with fewer interdependencies between components.
