Generally "back-to-back" also implies packets are not only 'consecutive, in succession', as @Georg Pauwen describes, but they are grouped such that there's no time lag between them, as shown in @wwjjj post. I.e.each packet is transmitted as rapidly as the medium will allow and they are, indeed, also physically, "back-to-back".
However, sometimes "back-to-back" might just denote they were transmitted "back-to-back", at source, but have medium gaps or other packets between packets of the same flow when they arrive at the destination.
An example of the latter, source transmits two packets "back-to-back" at 100 Mbps, but are delivered to the destination on a gig link. At source transmission might look like [....P1....][....P2....] but at destination [P1]........[P2]......... As gig takes 1/10 the time to transmit at FE, we now have time gaps between the packets, which could allow another packet, or packets, during that time. E.g. [P1]...[Q#]...[P2]......... NB: Q# is a different flow's packet interleaved with P's packets.
