SRP is very simple. If two different people want to change a class for two different reasons, then pull those reasons into two different classes.

That is the SRP.

Example: A class that analyzes a data stream and prints a report. The data analysis will interest one group of people. The format of the report will interest another, different, group. The first group will ask for changes to the algorithms. The second will ask for changes to the format. The principle says to separate those two concerns.

This goes all the way back to David Parnas and the separation of concerns. His papers that describe it are freely available on the web. I suggest that they be studied, because they are full of wisdom and insight.