A sewer clean out is an external access point to your home's plumbing system. Most often sewer clean outs can be found outside near the exterior walls of the home. These capped pipes are usually quite unobtrusive and unless you are looking for it you may not even notice it is there. Not all homes have a sewer clean out while some have more than one.
Having a sewer clean out is good for several reasons. Looking at the clean out can help verify if you have a sewer drain clog or not. This is important since a sewer main clog is a plumbing emergency whereas a fixture clog is not.
Also, the sewer clean out will make clearing a sewer drain clog easier and less expensive. A drain machine can be used at the clean out to clear a sewer clog. The alternative is to run the machine through an internal fixture or through a roof vent. Both of these alternate options are considerably more difficult and more costly if you hire a plumber to do them.
It is a good idea to determine whether you have a clean out or not ahead of time and locate where the clean out is to make things easier in an emergency. If you don't have a clean out and have frequent sewer main clogs you might consider having one put it. It can end up saving you time and money in the long run.