Pronouns are a basic part of language - it's that little word used in place of a persons name, and is usually "he" or "she". However, for someone who is outside of the gender binary, using he or she when referring to them can be very offensive and can cause the person dysphoria - which is never a good thing.
What if I don't know some ones Pronouns?
ASK. If you're unsure what some ones pronouns are, don't be afraid to ask. Simply pull them inside and ask them privately. You could save a lot of people a lot of pain by asking a quick question.
What pronouns are there?
Most cisgender people don't even think about pronouns or consider which ones they want, they typically just follow the one they were assigned at birth (he or she). However, there are also more gender neutral pronouns, the most common is 'they/them'. A lot of people have trouble using 'they' for their peers because they are confused by the gram mer, but after you get the hang of it, it will come naturally [ex: instead of saying "I'm taking he/her to prom", say "I'm taking them to prom". Instead of "She/He and i had a great time yesterday", say "They and I had a great time yesterday"]. if you re having too much trouble using they/them, you could just avoid using pronouns and say their name instead.