In a world of global communications, there is often a need to provide web pages for multiple laguages. The structure of the pages need not change, but the content and any messages, prompts, titles and other displayed content does need to change. MODX uses Lexicon files to provide i18n support.
Lexicons aren't intended for actual page content; while they can be used, it is awkward to edit and create new lexicon content often. There are other extras such as Babel, Lingua and migxMultiLang to help manage multiple-language resources and their content. Lexicons are useful primarily for more static displayed content, such as error messages, form field labels, footer content, anything that isn't going to change very often.
The core MODX lexicon files are in core/lexicons/. Each extra has its own lexicon files in core/components/component-name/lexicons/, as does the Manager in manager/lexicons/.
The Manager has a Lexicon manager where you can select the namespace, language and topic of the file to be worked with.