European boats, in particular, frequently use a spring-loaded terminal. Looking down from the top you may see a small indentation, which accepts the tip of a small screwdriver. Push down, and that releases the clamp and allows the wire to be pushed in.
There are other types that are similar (use caution, the exact style shown in the image allows metal-metal contact which means your screwdriver could end up with voltage, other styles can isolate the two and still work the same way).
There are other styles that have a little plastic lever (usually orange or yellow) that opens the terminal for insertion.