A sub is a version of an anime that keeps the original voice track and presents translation as subtitles. "Sub" names the format, not a quality tier: subtitled releases range from rapid fan work to carefully revised official editions.
Subtitles are the oldest bridge between anime and the rest of the world. Fan-made subs carried the medium abroad for decades before legal streaming, and the modern professional sub is their descendant: a timed text track produced at speed, against deadlines measured in hours, for same-day release.
Translation for subtitles is a chain of judgement calls. Honorifics may be kept or dropped, jokes rebuilt, names reordered to Western name order or left in Japanese order, and each choice trades accuracy against readability at twenty characters a second. Two official releases of the same episode can legitimately differ, which is why a fact that turns on a line's wording must name its release.
The sub-versus-dub argument is fandom's oldest campfire fight, and this site stays out of it by design. Both are legitimate doors into the same work; the glossary's job is to define the terms, and the canon sheet's job is to say which text a quiz is standing on. Professional subtitle work is credited and increasingly visible: translators are named on releases, and their choices are reviewed by audiences who compare versions with a scholar's attention.
In use"Watching with the original Japanese performances and English text at the bottom is watching the sub."