Your lateral is your sewer pipe that runs from your home to the city connection or a septic tank. In order to determine the source of your problem, I recommend having your sewer lateral video inspected to see if you have any offset joints, inadequate grade or slope, or complete separations. These repairs can be costly, but you do have options as these repairs can be completed using the trenchless method (pipe bursting) or CIPP (cured in place pipe), aka pipe lining.