1. People in general
   "They say it's going to rain tomorrow"
    
2. Plural of he, she, it
   "John called Mary, but they haven't spoken since"
    
3. He or she; generic third person singular pronoun, used to refer to a person or animal of either sex (this sense of 'he' is regarded as politically incorrect in some circles)
   "When you do find someone, they will have a tough time catching up";
   - he
    
4. Refers to single person whose gender identity is non-binary
   "Alex said they would be late"

Encyclopedia: They