The argument is that certain tools introduce complexity into a project by the mere act of using them. A power drill has many more knobs and features that the user must be aware of than, say, a screwdriver. Sometimes those features are required, but often developers will gravitate towards such tools even when they're not.

In software development this applies not just to external tools, but to abstractions in general. We might be tempted to create or depend on an abstraction that solves many problems in a generic way, when in reality this could be avoided. The appeal is to reject this temptation whenever possible.

Simplicity is an amorphous quality without a clear path towards it. It's often debatable whether a system is simple or complex, given that most software is built on extremely complex machinery we take for granted. But the least we can do is to be mindful of practices that lead to a net increase of complexity, use critical thinking instead of blindly following trends, and, sometimes, take the more difficult road for the sake of preserving simplicity.