At the end of every calendar year, popular English dictionaries each pick a "Word of the Year". In 2019, Oxford's was "climate emergency", and Cambridge's was "upcycling". Merriam-Webster, a dictionary based in the US, picked "they".
As you learned in English class, "they" is usually used as a plural pronoun, for example:
> I didn't see Sam and Janine. They didn't attend the conference. <
English generally uses "he" or "she" as singular pronouns. But this practice is a bit problematic because the words aren't gender neutral. What if someone identifies as the other gender (male/female), or neither of the two? What if you don't know if a person is male or female?
That's where "they" enters the picture. Using "they" as a singular pronoun means you don't have to assign a gender.
Some people also prefer to be called "they/them" instead of "he/him" or "she/her".
This evolution is a bit similar to a change in formal letter and email greetings that took place years ago. For example, rather than writing Ms./Mr., you might see "Dear Sam Thompson" -- with both first and last names and no title -- in the greeting. Although it might feel impersonal, it's gender neutral, with no risk of writing the wrong thing.
You might not see a singular "they" in your business correspondence this week, but don't be surprised if you see it sometime soon.