When I started working with Magento 2, that was the first time I came accross this abbreviation so I have no idea what does that word/abbreviation mean ?
Wikipedia answered my question:
Internationalization and localization
The terms are frequently abbreviated to the numeronyms i18n (where 18 stands for the number of letters between the first i and the last n in the word internationalization, a usage coined at DEC in the 1970s or 80s) and L10n for localization, due to the length of the words.
So if you ever wonder i18n means internationalization
i18n folder will use for translate content for multi language.
For eg. website have multiple language option so need translation for all content. and translation will put inside i18n folder in as CSV format.
"Save and Continue Edit","Save and Continue Edit"
"Save and Continue Edit","Save and Edit Again" and test
For translated content use like in frontend as
__("Save and Continue Edit")