First, it should be noted that “kudos” is not the plural form of “kudo”, so a “kudo” was once technically nothing. However, because so many people in the last century, mainly in the United States, have thought kudos was plural, in some dictionaries today “kudo” is considered a valid word meaning the same thing as kudos (yet another word created via back-formation).
To answer your question, kudos in English means:
Praise / Accolades or Credit for one’s achievements
The word “kudos” comes from the Greek κῦδος (kudos), meaning “glory” or “fame”. The “-os” ending in Greek typically indicates a singular noun and is supposed to be pronounced like “-ose”, rather than “-oze”, as many Americans usually pronounce it, “koo-doze”, or as a lot of British people tend to pronounce it “-oss”, “cue-doss”.
The word made its way into English around the late 18th century / early 19th century, meaning pretty much the same thing as it means today. The first documented instance of the “singular” word “kudo” didn’t pop up until 1926.