It's simply the process of taking a DI (stands for "direct injection") of the guitar while recording. Specifically, this means you record not only the output of the amp, but also tap into the signal coming directly from the guitar. So you record two tracks; one track of guitar amp, and one track of "just the guitar" - like if you had plugged your guitar directly into the interface. In reality, mostly you'd use something called a "DI box", which has a high input impedance to ensure the guitar signal gets "read" correctly. DI boxes have a "direct through" output, which you'd then plug into your amp via a cable. This "through" connection also carries the guitar output signal, so you can hear and record the guitar amp alongside with the DI track (note that you do not need a dedicated DI box with the kemper, you can set it up so one of the outputs on the kemper sends the "guitar only" signal to your computer).
I hope that was clear so far
Reamping, then, is the process of sending out this recorded DI track to an amp and then recording that amp. For this, you'd normally use the "inverse" of the DI box, which is called a reamp box (again, you don't need this when you have a kemper - see the reference manual, the chapter on "reamping").
This will allow you to re-record the SAME performance (as you have already recorded the "raw"/direct guitar track) with a different amp / amp settings / mic setup. More specifically, for us, it allows us to record the performance with a different profile - after the fact.
So if you've recorded something and you find out that you were unhappy with the tones a day after, you don't have to actually physically play it again.
On the flip side, having the option can lead to second-guessing and third-guessing (etc. etc.) the chosen guitar tone.