Everyone has friends who tell stories of seeing a Fairy, and everyone's cousin claims to have had one light on their hand.
In reality, it is hard to actually see a Fairy. Fairies will never just appear in front of you. Usually they can be seen as a glint or a flicker in the corner of the eye. The little motes in a sunbeam are Fairies, but the closer you look, the harder they are to see. Maybe you sense someone else near you, but when you turn you only see a vague outline of where they were.
Fairies take all forms from microscopic to large, but are always humanoid and always nearly translucent. When you catch a glint of light off the morning dew, or get a sense of wonder in a serene meadow, those are fairies dancing in the sunbeam.
There is great debate as to whether Fairies are actually aware of us or our plane or if they have any intelligence at all. There have been no records of any Fairy being seen holding or wearing any manufactured item, and their movements while deliberate do not seem to be guided. It is not known if they are aware of the real world or even if they are aware of other Fairies.
Fairies come and go between the planes at will but seem to be nearly completely oblivious to our world with one exception: they are drawn to beautiful things like gems, flowers, sunlight, art... Peaceful meadows in deep forests often teem with Fairies dancing in the sunbeams and among the flowers. Intricate gems will glitter with the effects of Fairies flitting in and out of existence near it.
Fairies can be found near all beautiful things, perhaps almost reaching out to them or dancing around them. And while they seem to avoid physical things in our world, it could be something more like how water flows around rocks not because the water knows to avoid the rocks, but more because that is just its nature.
While Fairies avoid physical things in our world, they can and will pass through them. They are more likely to move through non-living things than living things, and more likely to move through non-magic things than enchanted things.
When a Fairy visits a place in our world. It leaves behind a tiny burst of Magic that will spread out and infuse the area. While it is not a physical thing, people often use the term "Fairy Dust" to describe the effect. Like a residue of the presence of Fairies.