What’s the difference between a single-entry visa and multiple-entry visa?
A single entry visa allows you to enter a country once during the validity of the visa. When you leave the country the visa is no longer valid and you would be required to re-apply for another visa if you wish to visit that country again. A multiple-entry visa will allow you to enter/leave the country several times during the validity of the visa. In the case of a Schengen Visa you would be able to use a multiple-entry visa to visit other Schengen countries.