Tech plates are the predecessors to bumper plates. Before the technology and engineering existed to make high quality (light) weight plates from rubber (bumper plates), tech plates were the answer to hold a barbell at proper IWF height to start your first pull for the clean or snatch (or the deadlift for that matter).
So, tech plates are weight plates constructed of plastic that have a 17-18" diameter. Sometimes, there is a bit of a variation between brands. They typically are not made in weights higher than 25lb / 10kg. In fact, we have never been asked for tech plates in weights higher than 10lb / 5kg.
While they absorb the impact of a drop, they don't bounce like bumpers do- and they tend to be loud on the drop. Durability of tech plates is a strong point, but don't believe anyone who claims they're unbreakable. I've seen them break, although it takes quite a few thousand drops.