This one-minute video explains the differences between medical flexible spending accounts and health savings accounts. You can only have a health savings account if you are enrolled in a High Deductible Healthcare Plan (HDHP).
For Flexible Spending Accounts, the video says that when the year is over you typically lose what you don't spend; however, our FSA at Garrett enables employees to carry over up to $500 from one calendar year (Jan-Dec) to the next calendar year. Since you have from January to March of the next calendar year to apply expenses from the last calendar year to that year's balance, the up to $500 balance from a prior year does not carry over until early April of the next year.
For example, at the end of 2020 you have $300 dollars in your FSA. You have from January to March 31, 2021 to still submit 2020 medical expenses against that balance. If by March 31, 2021, you still have a remaining balance from 2020, that balance will be transferred in early April to your FSA 2021 balance.