Most European countries are part of the Schengen area, in which there are no external borders, and people can move freely among countries. When the UK was part of the European Union, UK citizens could spend any number of days in the Schengen area.
Now that the UK has withdrawn from the EU, UK citizens can only spend 90 days in any 180-day period, in all Schengen countries combined.* For example, if you have spent 90 days at a second home in Spain, you’ll have to wait another 90 days before you can visit friends in France.
UK citizens don’t need a visa to enter the Schengen area. However, the European Union is currently implementing a waiver programme called European Travel Information and Authorisation System (ETIAS). It is designed to pre-screen travellers who don’t need visas. ETIAS should become mandatory by the end of 2022. At that point, you will have to apply for an ETIAS to enter the Schengen area.
* We have simplified this explanation. Rules vary greatly if you are a UK citizen planning to become resident in a Schengen area country.