The difference between digital and analogue reproduction lies in the peaks and troughs of the sound waves. If you just take an analogue tape and transcribe it "as-is" to a digital medium, the highs and lows are "clipped" which takes a good deal of the life out of the recording - it makes each sound rather disjointed, almost as if there is a pause in the middle of each sound.
Competent remastering involves packing all of the sounds fromt the analogue source within the usable range of the digital medium, a kind of compression of the sound. It restores the continuity, although it still leaves the digital recording sounding somewhat flatter than the analogue. Go and compare your favourite LP with a remastered CD of the same music. It will still sound like rubbish in comparison.
I read a brief description of how it's done ages ago, and this explanation comes from that. The technical hoopla involved is beyond my ken!
