The Schengen Area is a bloc of European countries with no internal border checks. Visa-free visitors (including Americans, Britons, Canadians, and Australians) can stay up to 90 days in total within any 180-day window — and crucially, that's across the whole area combined, not per country. Days spent in France and Spain count toward the same allowance.
The window is rolling, not a calendar reset, so the count looks back over the previous 180 days on any given date. Overstaying risks fines and entry bans, which is why long-term movers need an actual visa or residence permit rather than relying on tourist entry.
Why it matters for your move
The 90/180 rule is the wall most people hit when they try to 'just stay a while' in Europe — understanding it is what turns a long European stay from a risk into a plan (and points you toward the right long-stay visa).