By clicking this, you can deconstruct cards you already own and break them back down into their Craft Points. Back in the deck menu, next to the generate button, there is a red button that says 'Dismantle'. By completing these missions, such as finding a Secret Pack or winning a duel in solo mode, you'll unlock Craft Points quite quickly. In the missions window, there are two tabs: Limited-Time Missions, which reward Gems, and Unlimited Missions, which reward more things including Craft Points. The second way is by completing missions. As you play, you'll unlock more and more levels and get more craft points. Though cosmetics and gems tend to be the most interesting rewards on the pass, there are also a lot of Craft Points of each rarity on offer. As you complete duels and level up, you'll progress through the Duel Pass and unlock various rewards. The Duel Pass is Master Duel's take on a Battle Pass.
There are three main ways of acquiring Craft Points: ranking up the Duel Pass, completing missions, and dismantling cards.
Craft Points are the currency used to generate cards in Master Duel.