Relining repairs a damaged pipe from the inside rather than replacing it from the outside. A resin saturated liner is drawn into the existing pipe, positioned over the damaged section, and cured in place so it hardens into a new pipe within the old one.
Relining is well suited to cracked sections, joints that roots have entered, and lengths where the pipe is structurally sound but no longer sealed.
It is less suitable, or not possible, where a pipe has completely collapsed and there is no longer a path to draw a liner through, where the line has severely deformed, or where the fall is wrong. A grade problem is not fixed by lining, because the liner follows the existing pipe. If a section sags, it will still sag afterwards.
This is why a camera inspection comes first. It establishes whether the pipe is a candidate at all, which section needs treatment, and whether the underlying problem is damage or grade. Anyone quoting relining without inspecting the line first is guessing.
If water is entering the house faster than it can drain, turn off the mains water, keep clear of any electrical fittings that are wet, and call for help. Do not try to clear a surcharging sewer or stormwater pit yourself, because the water can carry contamination and the pit may be deeper than it appears.