Gendered language refers to any form of language which implies the gender identity of the person it is referring to. Using gendered language which does not match someone's gender identity is a form of misgendering.
How gender is expressed varies between language - some languages are more heavily gendered, while others are completely gender neutral.
Common examples of gendered language in English include names, pronouns and titles. However, some nouns also imply gender, such as "brother" (male) vs "sister" (female) or "chairman" vs "chairwoman". English often uses male forms of words as the default, which is considered sexist.