Used to generate translations from __('string to translate', 'text-domain') functions.
MO files: MO, or Machine Object is a binary data file that contains object data referenced by a program. It is typically used to translate program code, and may be loaded or imported into the GNU gettext program.
PO files: PO files are the files which contain the actual translations. Each language will have its own PO file, for example, for French there would be a fr.po file, for german there would be a de.po, for American English there might be en-US.po.
POT file: POT files are the template files for PO files. They will have all the translation strings left empty. A POT file is essentially an empty PO file without the translations, with just the original strings.
wp i18n make-pot . languages/<file-name>.pot --domain=<domain-name>
wp i18n update-po languages/<file-name>.pot languages/fr_FR.po
wp i18n make-mo languages/<file-name>.po languages/Sources: