One of the older virtual realities, but still strong after ten years, this is a hard place to describe. For me, it is a place where I go to experience a different reality - I 'wear' an avatar who can go to classes in virtual building and scripting, terraform virtual land and build virtual objects. When my physical travel in Real Life was restricted, I was able to turn on my computer and visit virtual representations of places like Mt. San Michel and the Great Wall of China, and also visit entirely new worlds where artists have built environments which would be impossible in places restricted by the laws of physics. Visiting Second Life is free, but there is a steep learning curve and you may encounter 'griefers' who like to bother newcomers.
Kolor Studio in Second Life - a constantly changing environment. Each time I visit it, it is different.