Short Answer: Subject-verb agreement is just making sure both subjects and verbs are both plural or singular. If you mix them up, it wouldn't sound grammatically correct. So basically..
A singular subject needs a singular verb.
Example:
1. Taylor Swift sings her song "Fearless" in the Eras Tour.
("Taylor Swift" is singular. "Sings" is singular too)
2. Joe Alwyn rides his scooter.
("Joe Alwyn" is singular. "Rides" is singular)
A plural subject needs a plural noun.
1. The children love to play badminton.
("Children" is plural. "Love" is plural too)
2. Pat and Jen play games together.
("Pat" and "Jen" are two people, so plural. "Play" is plural)
TLDR:
Basically for anything that's singular, just put an "s" or "es"
Don't do it for plural :)