A cache line is simply the unit of data a CPU cache works with (generally 64 bytes, because someone somewhere has probably determined that that is the best line size for general use), much like there are units of data like bytes (8 bits nowadays, but there have been weird ones historically), pages (varies between hardware as well, and may be OS-configurable), etc.

As TFA mentions, a CPU does some predictions about what cache lines to prefetch, e.g. when you do sequential reads. Moreover, the x86_64 instruction set provides a prefetch instruction through which you are able to give the CPU a hint "hey, I'm gonna be using this soon, prepare accordingly, pretty please".

Still, the utility of prefetching is diminished if you only use a single byte from each cache line, because the mechanism generally depends on you doing other work while the next cache line is being fetched. So really the best case scenario is to take as much time as possible to work with what is already fetched, so that there is time for the next unit of data to be fetched in the meantime.