Off-line switchers convert a usually high AC input voltage (e.g. 240VAC mains supply) down to a IC level DC voltage (e.g. 5V). Their design is based on a flyback regulator circuit.
The term “off-line switchers” comes from the fact that the DC output voltage is derived straight from the AC input (the DC voltage comes straight “off” the line (AC)).
IMHO, this was a bad name choice, as many people would assume it is related to being offline (disconnected), rather than online. Remember back to when dot-matrix printers had a “online” LED?
Both isolated and non-isolated off-line switchers exist on the market.
And here is an example of a non-isolated version: