Typically, society has taught us to make automatic assumptions about what pronouns to use for someone. If a person’s gender expression (the way they appear in terms of gender) seems to be male, we’d likely use he/him/his when talking about that person; if a person’s appearance seems to be female, we’d be likely to use she/her/hers. However, gender is not always that simple. Sometimes a person’s gender identity (the way the person identifies internally in terms of their gender) doesn’t align with their gender expression (the way they look). In addition, not everyone identifies strictly as male or female. So when a person includes their gender pronouns on their email signature line (or on a nametag, when introducing themselves, etc.), they are simply taking the guesswork away for you! It’s their way of saying “when you refer to me using pronouns (opposed to by my name), these are the pronouns I’d like for you to use.”
For more information: https://www.bottomline.org/content/support-gender-inclusive-pronouns