Sub

A sub (also known as "Subtitle") in anime usually refers to a process that preserves the original soundtrack voices and adds a text translation in the video somewhere, usually at the bottom. It is often mentioned in contrast with a dub which uses a voice soundtrack in a different language from the original Japanese language voices.

Why you should watch anime with Japanese audio and subtitles
Most anime are authored by Japanese creators and made by Japanese producers. Often they include Japanese culturally specific aspects like different regional dialects, historical references, rituals, and behavioral cues.

The casting for the best anime takes into account these many cultural issues and so do the voice actors. The pool of Japanese voice actors is large and skilled enough and the experience of sound directors has built up to the extent that even American dubs with decent money can't begin to compare with the quality of voice acting in Japan.

If you watch an English dub (it's probably similar or worse for other non-Japanese languages), you're usually hearing the same 50 or less voice actors that can actually make a living in the much reduced dub market. The choices are more often driven by money (see the many articles about massively underpaid western voice actors) and speed with quality being very low on the priority list.

How do you think English (or other non-Japanese) dubs handle the difference between Kansai and Kanto dialects that occur frequently in anime and often are part of the story? They mostly don't. What about the puns and jokes based on Japanese language? In subs, there is often an explanation in parentheses, but in dubs they almost alway skip over it and just use a wonky translation that they hope the audience just skips over as something weird and inscrutable.

Then there's the music. Most dubs never change the music (again usually due to money), but many subbed anime give subs for the music as well. It is super extremely rare for dubs to actually replace the music with something in the dubbed language even more insanely rare for it to be of similar quality.