Pronouns are linguistic tools that we use to refer to people. (i.e. they/them/theirs, she/her/hers, he/him/his). It is important to give the opportunity for people to state the pronoun that is correct to use when referring to them. Often times, people make assumptions about the gender of a person based on person's appearance and/or name. This can be very harmful in stating that people have to look a certain way to demonstrate the gender that they are or are not. Using personal pronouns is a way to respect others and create an inclusive space just as using a person's name can be a way to respect them.
(This is NOT an exhaustive list)
Using pronouns other than the ones a person has asked you to use can make them feel unwelcome, invisible, or unsafe in the community. This is called misgendering someone. Sometimes a person's pronouns will change without a change in gender identity or expression. If you make a mistake, quickly correct yourself and apologize. Do not make a big deal out of it for it is not necessarily helpful and could be harmful.