What does it mean to "isolate a circuit"?

using transformer isolate circuit means reference, e.g. ground, on primary side no longer applies secondary side. once done, use of transformer, can ground single point wish ground, on secondary side of transformer , circuit. safety increased because electrical plugs can reversed, incoming wire may have been connected ground, , chassis, connected "hot' (non-neutral or ungrounded) wire of plug, when plug turned around. if occurs chassis connected 110 vac (220 vac in of world), resulting in unsafe, , potentially lethal, condition.

when use transformer , isolates circuit, mean? , safety advantages of it? how make safer?

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