Your first floor and basement are below the elevation for the sewer. That means waste must be pumped up to the sewer for disposal. From your post, I could not tell if you are having a problem with the system, but It consists of the grinder pump that is activated on a float system, and a one-way valve somewhere that keeps sewage from backing down from the main to your home. Absolutely critical that this system operate properly. Do a Google search on sewage pump and learn about them and be sure you know where to get spare or replacement parts. Also, consider a backup generator if you are in an area subject to frequent power outage.