A trenchless repair relines an existing pipe from the inside, using a flexible liner coated in resin that is pulled or inflated through the damaged section and left to cure into a new pipe wall within the old one. Access typically comes through an existing cleanout or a small excavation at each end, not a trench along the whole run.
This works well for a line that has cracked, separated at a joint, or lost material to corrosion, as long as the pipe has not fully collapsed. A collapsed section usually still needs to be dug up and replaced outright, since there is no intact pipe left to line.
A camera inspection is what decides which category a given line falls into before either approach gets scheduled, rather than choosing the method first and hoping it fits.