There are many countries in Europe that are comparatively smaller and people earlier needed to get visas for each country they wanted to visit.
Therefore, an agreement was signed in a town called Schengen where three countries initially agreed to provide a single visa to visit any of their member countries. That list currently has expanded to 27 countries: Austria, Belgium, Croatia, 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, Switzerland.
Getting a Schengen visa allows the travelers to visit any of the member countries in the Schengen Zone as there are no border controls among the member countries. You just clear the immigration and customs in the first country you enter in the Schengen Zone. Following that, you can travel to any of the member countries in an airplane, train, bus, taxi or driving, just like traveling domestically within the country, all on the same visa. That makes it quite convenient avoiding the hassle of visas for individual countries.