As was the case with Flash Gordon, there is more than one film version of the project to consider. It all starts with the 1951 novel by Sf author John Wyndham, who also crafted for us the classic tale The Midwich Cuckoos later made into a fine film title The Village of the Damned.
Triffids was adapted into film three times, in 1962 as a feature film starring Jeannette Scott, in 1981 and 2009 as television limited a series each time. I have seen the feature film and the 1981 productions but I have yet to view the 2009.
The 1962 feature film is the production most people are familiar with. In the film the Earth is treated to a rare comet that produces a dazzling lightshow during the nighttime hours. Nearly everyone on the planet turned out to watch the event. The light show had a disastrous side effect; it renders everyone who directly viewed it blind. Our main character is a sailor being treated for eye injures and as such was spared the nearly universal blindness. In addition to burring out everyone’s optic nerves the comet also brought spores for a new plant species; the Triffids.
Triffids are mobile, carnivorous plants. They move slowly and are able to spit poison. Had the population not been blinded they would have presented a minimal threat, but robbed of sight people become easy pickings for the predators. The film follows a number or survivors as they battle triffids, each other, and their own inner demons.
The 1981 television version was a little closer to the original novel. The planets were primarily the result of genetic engineering, a crop meant to create a new supply of oil and hydrocarbons. I honestly cannot remember what caused the planet-wide blindness in that production. In the novel it was not a comet, but rather a malfunctioning orbiting weapons system that blinded the world.
The Day of the Triffids is worth seeing at least once so you can punch your geek cred card, but the film is too flawed for repeated viewings.