Okay kiddo, so you know how when we talk, we sometimes change the endings of words to show if we are talking about one thing or many things? That's kind of what archaic Dutch declension is all about.
Back in the olden days, people in the Netherlands used to change the endings of words even more than we do now, to show things like who was doing the action or who the action was being done to. They had different sets of endings for different groups of words, kind of like special rules for each group.
So let's say we have the word "kat" (cat). If we wanted to talk about one cat, we would say "de kat" (the cat). But if we wanted to talk about many cats, we would say "de katten" (the cats). See how the ending changes from "-t" to "-ten"? That's declension.
But it gets even trickier! In archaic Dutch, they also had different sets of endings for different genders of nouns (which is just a fancy way of saying whether the noun is male, female, or neutral). So if we had a male cat, we might say "de kater" (the male cat), but if we had a female cat, we might say "de poes" instead (the female cat). And the endings we use for each gender might be different too!
Overall, archaic Dutch declension can be pretty confusing, even for grown-ups who speak modern Dutch! But back in the old days, it was an important way for people to communicate clearly and unambiguously.