The envelope is thick. That is the first clue.
Inside is a letter with your name on it, a number that looks enormous, and a deadline roughly 60 days out. Somewhere in the packet are phrases like "one time opportunity" and "election window" and "this offer will not be repeated."
Congratulations. You have received a pension buyout offer.
Here is the part the letter will not say plainly: your former employer is not doing this for you. They are doing it because getting your pension off their balance sheet is worth money to them.
That does not automatically make the offer bad. Sometimes it is a fine deal. But you should evaluate it knowing that a room full of actuaries priced this thing to work out in the company's favor on average.
So let's take the packet apart. What a buyout actually is, why companies offer them, the one calculation that tells you most of what you need, and how to figure out whether you are the person who should say yes.