FAA requires that there needs to be an exit per a certain number of passengers. More passengers, more exits.
The 737 already has 8 exits, two forward doors, two rear doors, and four overwing exits.
If an airline wanted to outfit their 737-9 with more than, but don't quote me I'm not going to look up the exact nubmer, 178 passengers, then two additional "mid-cabin exit doors" are required to certify the aircraft for the additional passengers
But the mid-cabin doors take up half a row of seats, takes away a row of overhead bin storage, and provides only a tiny porthole of a window for the passenger (see pic below.) If an airline configures their interior to be below the magic number (I guessed 178), then the airline doesn't need to have the two mid-cabin emergency exit doors. In this case a plug is installed in place of the exit door, and a full sized window is built into the plug. A fully finished interior wall covers the plug location and an overhead bin is installed, so to the passengers it just looks like another window. The only way to know you're sitting in a 'door plug' row is that the spacing between windows is slightly wider where the plug is, and on the exterior you can still see the outline of the door.
Also, the plug is slightly lighter than the door and its automatically deploying slides. So if you don't need the door, you save a bit of weight with the plug as well.
If at any time in the future the airline wishes to reconfigure the cabin for maximum passengers, they can remove the plugs and install emergency exit doors because the framing is already built into the fuselage.
Mid Cabin Exit Door:
