SSAs are special assessments areas that builders cooperate with towns to get in order to pay for infrastructure improvements like water and sewer connections, street lights, etc. They are structured differently sometimes, but the ones I have seen essentially put an extra assessment annually on the homeowner for a period of time - 20 or 30 years typically. Sometimes they can be as much as an extra tax bill or sometimes they can be lower.
An SSA must be disclosed by the seller and is supposed to be disclosed on the MLS. You must do your homework on these as they can be a lot of money or a little money and can be properties less attractive.