It describes a world where a strange plant, the `Triffid' of the title is a major crop, due to its invaluable edible oils and proteins, even though it has some major disadvantages for a crop plant - a degree of mobility, some rudimentary animal like reflexes, and a whip-like poisonous sting (!) - though the sting is normally removed in cultivation.
The book opens with the hero in hospital, with his eyes bandaged after having been hit by an uncropped triffid, discovering that while he had been blindfolded, an unusually bright meteor shower that had been in the news (or possibly some space-based weapons system) had blinded most people on earth. The handful of sighted survivors escape the general collapse, to be faced by the growing numbers of free, undocked triffids...
Later made into a film, in which the triffids arrived as spores in the same meteor shower that blinded almost everyone on earth, which at least simplified the number of assumptions required to make the plot work.
The book was also adapted as a limited series for English television.