A belly in a sewer line is a section that has sagged below its proper downhill grade, forming a low spot where water and waste collect instead of draining away. It is a slope problem, not a hole in the pipe. Because the dip holds standing water and solids, the line clogs in the same place again and again.
Bellies come from the ground moving, not from the pipe wearing out. Soil that was poorly compacted under the pipe settles, or, across North Texas, expansive clay swells and shrinks with the wet and dry seasons and drags a length of pipe down with it. Rigid older materials like cast iron and clay tile give the least, so the movement shows up as a sag or a pulled joint.
On a camera the belly looks like a pool of standing water sitting in the line between drains. The symptoms a homeowner notices are recurring clogs, slow drains, and backups that return no matter how often the line is cleared. Hydro jetting washes the settled sludge out, but it cannot raise the pipe, so the low spot fills again. A camera inspection measures how deep the water sits and how long the sag runs.
Pipe lining does not fix a belly either, because a liner cures to the shape of the pipe it is inside and simply follows the sag. The lasting repair is to dig out the bellied section and re-lay it on firm support at the right fall, which is covered in whether lining can fix a belly. One honest caveat: there is no fixed line between a minor and a severe belly, so a shallow sag in a sound pipe can sometimes be managed with periodic cleaning.