A Schengen visa is a short stay visa allowing its holder to circulate in the Schengen area. The Schengen area covers 26 countries (“Schengen States”) without border controls between them. These countries are: Austria, Belgium, the Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Italy, Latvia, Liechtenstein, Lithuania, Luxembourg, Malta, the Netherlands, Norway, Poland, Portugal, Slovakia, Slovenia, Spain, Sweden and Switzerland.
A Schengen visa is an authorisation issued by a Schengen State with a view to:
- an intended stay in the territory of the Schengen States of a duration of no more than 90 days in any 180 days period (“short stay visa”),
- a transit through the international transit areas of airports of the Schengen States (“airport transit visa”).
https://ec.europa.eu/home-affairs/what-we-do/policies/borders-and-visas/visa-policy/schengen_visa_en
The UK is part of the Scheme, hence the 90 days in 180 applies. Staying longer than 90 days in 180 will mean you will be required to apply for a visa
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