A continuous block occurs when you cannot intentionally let go of a block at any time just by releasing the directional input. This will cause you to continue the blockstun from blocking the previous strike hitbox until you block the next one.
Without this, things like (blocked) combo attempts and multi-step moves have a much better possibility of being stuffed. On the upside, this makes guard cancels and some kinds of counterattacks a bit easier to time.
Notably, the Street Fighter Alpha series has a continuous block system. Street Fighter III had a continuous block system in New Generation and 2nd Impact, but ditched it in 3rd Strike so that players could attempt parries from out of a normal block (and be punished if they failed).
(Finally, as a matter of trivia: in KOF ’94 and KOF ’95, which have continuous block systems, most players know that you can guard cancel when the meter is charged to MAX; however, it is also possible to do this if you block seven or more successive hits. The main problem with this, of course, is that very few moves do this, so it rarely pops up. But that is also why hardly anyone knows that.)