Localization, in general, is the process of adapting your product to a target market’s language and culture. The same applies to React apps.
It is the process of adapting your React app to another country/region with specific language and characteristics to meet the target-market standards.
You might be interested in: 
How to: Localise your React App with LinguiJS and Localazy
How to localize React app with react-i18next and Localazy