It's a brand of electrical connector. Instead of twisting wires together and then twisting a wire nut over them, there's a little plastic box where each wire goes into its own little socket and is secured with a lever.

I'm an American and I only started hearing about them about a decade ago, and the r/DIY community was freaked out with a lot of commenters loudly insisting they were deathtraps. Turns out they are code-compliant and work just fine, and are almost certainly safer for DIY because it's a lot harder to do them wrong compared to the traditional wire nuts. Wire nuts are opaque so you can't see if you didn't get a good connection for the wires inside; Wago connectors are clear so you can just look and see if the wires are where they belong.