The apostrophe (that’s this little sign here: ‘) is normally only used to show possession or omission.
This is my colleague’s office. (possession)
We watched the boys’ football team. (possession)
There’s a lovely lamp on her desk. (omission)
They grew up in the ’90s. (omission)
We don’t normally use apostrophes to form plurals.
We bought eggs, mushrooms and peas in the supermarket.
This book was written in the 1990s.
However, apostrophes can be used to avoid confusion or misreading:
She received two A’s, three B’s and two C’s.
I hope that helps!