The single-responsibility principle is somewhat self-explanatory. It means that, in programming, everything — e.g. a class, function, module, plugin — should do one thing.
Some benefits to this approach include:
Because the idea is for code to stay focused on one thing, I often describe this principle as a chunk of code should do one thing and do it well. I say that because I find an implication that by breaking up code into individual responsibilities, the resulting code should be better — more solid and stable, easier to read and understand — than it would have been if responsibilities were combined.