Most countries have fairly straightforward visa (or visa-free) arrangements↗,where an entry stamp serves as permission to stay in the country for X amount of time. A month, 90 days, whatever. The Schengen Area↗ is different. Encompassing 26 European nations, the area abolishes most passport controls between the member countries. This means that, once a visitor has permission to enter one Schengen country, they can freely travel to any other as long as their visa is valid. But there is a catch. The terms of the visa are more complicated than most countries, and make it basically impossible to repeatedly renew a travel visa (for example by visa run↗).
A Schengen visa permits guests to stay for only 90 days in a 180 day period. This means that a three month stay must be followed by three months away before a visitor will be allowed to re-enter the area. There are other configurations that could work (one month on, one month off, rinse and repeat), but for anyone considering a long-term nomadic lifestyle, these limits provide a challenging barrier to extended stay in Europe.
Alternatives that would allow longer stays (student visa, work & residency permits, etc) would require picking a country and settling down, so as long as we want to keep moving, we have to keep Schengen-hacking (which is a dorky way of saying we have to leave sometimes).