You're grading decks on how much practice/experience you need with them or with similar decks to be able to play the deck satisfactorily in a tournament.

1 is you can pick this deck up the day before the tournament without ever having played it before and you’ll be able to perform at a satisfactory level.

5 is you should not take this deck to a tournament unless you’re very experienced with it.

This should include how easy it is to grasp, how intuitive the mulligan, sideboarding and in game decisions are, how hard it is to play perfectly, how punishing it is when you don’t play perfectly, and so on. If for example there’s a deck that you believe is very hard to play perfectly but that doesn’t require you to play perfectly at all to be able to win, then that would be an easy deck to play (even though it’s in theory very hard to play perfectly).

So, remember, this isn't intrinsically "how complicated is this deck?" but "how much practice do you need before you take it to a tournament".