After a plaintiff files a civil complaint, they have to properly serve all defendants. A summons is the document issued by the court that will be served with the complaint.
The summons itself is the document that triggers your obligation to respond to the complaint. You have twenty days from service of the summons to submit a response- though this period is often extended by agreement, you should not wait ton contact an attorney so that they have time to advise you of the most appropriate response, and to prepare it.
Some things you should know:
It is the date you are served, not the date on the complaint, that starts the 20 day period.
Even if you know about the complaint from another source, you do not have to respond until you are properly served.
It is common in Massachusetts for the summons and complaint to be served by the Sheriff's Department. This can be alarming, but it is normal.
If you do not respond to the complaint, you could lose by default.
Learn more here about what to expect as a defendant in a civil lawsuit.