A multilingual or multi-language website is any website in two or more languages. Typically they will have similar content in each language with a language switcher to choose your preferred language.
A site might offer language selection and then present the content in only a single language at a time. A multilingual site might also mix multiple languages within the same page, either because the audience is believed to be multilingual, or because there might be a need for embedded foreign text.
Having a multi-language site shows your customers that you are culturally sensitive.