其【qí】

其 before a noun means "its", "his", "her" or "their". It points back to a person or thing already named earlier in the sentence. It is a written word.

In speech the same idea takes a pronoun with 的: 他的, 它的, 他们的.

• × 我认识小雨，其妈妈是医生 → 我认识小雨，她妈妈是医生

Only before a noun

其 never stands on its own as the subject of a sentence. Where English would say "it" or "they", use 它 or 他们.

• × 这家公司很大，其有三千人 → 这家公司很大，有三千名员工

及其

及其 is "and its" or "and their", and you'll see it often in notices.

其 also lives inside several common words – 其中 "among them", 其他 "other", 其实 "actually", 尤其 "especially". There it is part of the word, and you learn each one as vocabulary.