Kingship, as Malcolm tells Macduff in act 4, scene 3, entails the possession of a number of very important qualities. Malcolm lists these qualities as justice, verity, temperance, stableness, bounty, perseverance, lowliness, devotion, patience, courage, and fortitude.
One senses that Malcolm didn't just make up these qualities on the spot; he's drawing upon a wider cultural understanding of what makes for a good king. And it's precisely because Macbeth so clearly doesn't possess any of these exalted qualities that Malcolm, Macduff, and other nobles of the Scottish court have escaped to England, where they are now devising a plan to overthrow the tyrant king.
Kingship is important because Scotland is a monarchy. The king is firmly in charge of the country, and therefore it's all the more important to have someone on the throne who can rule wisely. Duncan was certainly a wise ruler, and the Scottish people loved him for it. But Macbeth, who murdered Duncan, is almost the antithesis of his predecessor. He embodies none of the qualities that Malcolm thinks makes for a good king.
As the kingdom of Scotland isn't a democracy, when the king shows himself incapable of ruling wisely, there's no alternative but for his subjects to rise up in rebellion and force him from the throne. And that's precisely what happens here. But such rebellion comes at the cost of instability, which makes it all the more urgent that the people of Scotland have a wise king on the throne once more. Only then will the nation be able to enjoy some measure of stability again.