Core principles of universal design, as defined in 1997 by the Center for Universal Design at North Carolina State University, mean that universal design is flexible - door handles, light switches, faucets, and so many other features can be operated by various heights, abilities, and ages without doing it perfectly. They still operate and do what they are supposed to do even if someone grips or touches it a little off-center.
Along with flexibility in use, universal design features have a tolerance for error. This means that a wide hallway or doorway still works without someone needing to guide themselves precisely down the middle without veering to one side or the other. It means that a rocker light switch still turns on if just a small portion of it is touched - and maybe not even touched by a hand but by an elbow, fist, or even an object being held.
The idea of universal design solutions just blending in where they are used means that they don't call attention to themselves as being there for a particular purpose or need and they essentially are invisible as far as being anything special.
This leads to the best thing about universal design - that it is intuitive. When a particular feature is called for or just seems to fit, we can just add it as long as it serves most people and doesn't stand out as being anything special except attractive design. Universal design is thoughtful and good design without a lot of complication.
If something looks like it should be done it can just be created or installed without trying to determine who it might appeal to if it is done - a vertical grab bar near the entrance to a tub or shower to provide occasional support or balance, a folding seat or built-in bench in a shower for those times when one needs or wants to sit while they shower, a lever door handle that makes it so much easier to use even when one's hands are full or dirty (or wearing bulky mittens or gloves), or a wider doorway (with or without a physical door of any kind in the opening) is called for to move furniture or people through it easily.
Universal design enables most of us to function well in a home environment - ours or someone's home we are visiting - without needing to know in advance how things work, being able to use them whether we are seated or standing, knowing how to operate something or use it whether we have been in that space or used those features previously, and not needing to possess any particular degree of physical strength or dexterity to use something.