I'm just reading the rulebook and I don't get the idea behind this sentence: "You don't refill your hand to six cards at the start of your turn"
The full text is this one: "Each player draws a card at the start of their turn. As the same goes on, your hand will shrink or grow. You don't refill your hand to six cards at the start of your turn"
Why six? Is there a hand limit?
There is no hand size limit in the game, see Page 3 of the rules "5. Spend 1 Action to draw 1 card". It states there that there is no hand size limit.
The reason it says "don't refill your hand to six cards at the start of your turn" is because a lot of games such as this very often have some sort of rule that tells you to "draw to X cards" at the start (or end) of your turn.
The sentence you quoted isn't really needed, it's just there as a reminder.
The reason for six? The game set-up tells you to draw five cards and then the turn sequence (step 2) tells you to draw a card. So on the very first turn of the game, you would indeed have six cards in hand.
The "You don't refill your hand to six cards at the start of your turn" comment is there just as a reminder that this isn't always the case.
Hope that made sense.