Condo fees in Massachusetts are the monthly amount that each unit owner pays for the joint expenses of the condo association.
    Ideally, the condo trustees meet each year, adopt a budget that includes enough to pay the anticipated yearly expenses plus a reserve, and send notices to each unit owner.
And in the perfect world, the budget is exactly on target, there are no unexpected expenses, and all the unit owners pay their condo fees on time.
The reality is usually something different, and it works well until suddenly it doesn't.   Here are two examples.
Some lenders won't approve a mortgage on a condo unless the reserves are sufficient.  This could affect you if you want to sell your unit, because your buyers won't get their mortgage and won't be able to close as you expected.  even if you're not planning to sell any time soon, you may want to refinance, but you can't get a new mortgage because the condo's reserves are insufficient.
What happens when your neighbors stop paying their condo fees?  If the condo association has a properly adopted budget, and has complied with all of the legal requirements, the condo association - not you or the other unit owners  - can get ahead of your neighbors' lender for some of the unpaid condo fees.  Since lenders don't like to lose their priority, they sometimes choose to pay their borrower's back condo fees and collect it from the borrower later.
This is not a solution to try at home.  Strict compliance with all the legal requirements is the minimum, and the requirements can be very detailed and precise.  Consult an experienced real estate attorney who is familiar with condominiums and their governing documents.