Five design rules, one per letter, all aimed at code that people keep changing.
Single responsibility: a class should have one reason to change.
Open/closed: you should be able to add behaviour without editing code that already works.
Liskov substitution: a subtype must work wherever its base type does, with no surprises.
Interface segregation: several small contracts beat one fat contract that forces empty methods.
Dependency inversion: high-level code should depend on abstractions, not on concrete classes.
Teams bother because each letter names a specific pain they have already felt. Shotgun edits across five files for one change. An if chain that grows with every new feature. A subclass that throws from a method it inherited. Stubs full of empty methods. Logic that cannot run without a real database. Applied blindly, though, SOLID produces a class per line of logic, which is its own mess.
Rewriting in plainer words…
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