Can someone remind me again about what is knot? (Not rope knot)

Google wolf/dog knot

Penis base gets swollen (hence knot), you can't take it out without both of the people getting hurt if it's already in, truck load of semen, penis back to normal

It’s something that actual canines do while mating to ensure their semen impregnates the female.

Basically the bottom section of the penis swells up and it prevents separation till they finish.