IIRC replacement of all friction surfaces ie all jugs, pistons/rings, bearings, cranks, valves/seats, etc - anything that has a wear component built in - with new, or used products that are within new specs, whilst keeping compnents that do not have an intrinsic wear pattern such as conrods, cylinder heads, and crankcase, on spec (subject to rigourous inspection though). Basically an almost "As new" motor.
"Zero timed" is where the core (crankcase only) is used and all components are brand new on top of that.
Happy to be corrected if this is not right (been a while...)