Who sent you the check?
Why did they send it to you?
What year is it for?
Why did you get the payment in 2019 instead of in the earlier year?
Will the payer send you a W-2 or 1099-MISC reporting the payment? They have until January 31 to send it to you.
If it's from a current or former employer, are you or were you a W-2 employee, or were you an independent contractor?