Effective interfaces are low-friction
It takes brainpower to understand interfaces. If attention is limited, then the attention-usage necessary to understand an interface diverts resources from other tasks, such as deciding how to use an interface or actually using it.
The concept of “low-friction” interfaces is just a metaphor: two objects that interact will experience friction insofar as their respective surfaces are self-inconsistent.
For a user to be self-consistent, they must understand what they need, why they need it, and how to use what they need to accomplish some goal. These things are not always true, but they can become more true if an interface is effective. If an interface is built for an audience of “newbies”, the interface needs to guide its users with education. If an interface is built for an audience of technical-workers, the interface should behave as members of that technical field should expect. So, in order for the user to be self-consistent, the interface must understand its audience and target the user correctly.
For an interface to be self-consistent, it should follow good design principles such as standard naming conventions, high-cohesion/low-coupling, Interface concepts should be atomic, etc. It should not introduce noise or interfere with a user’s understanding. It should work as expected, with little need to explain itself. It should present specific, scoped utility, and not prioritize flexibility above clarity… though, flexibility is not a sin.