If you have a knack for doing something, you do it well. You may have a knack for baking cakes or a knack for cracking jokes. These things come easy to you.
Although the noun knack refers to a special talent, the sense is that it is a more innate talent — as opposed to a certain technique. So if someone instinctively has a talent for fixing plumbing, for example, you can say they have a knack for it. If, however, they unclog a sink by having a wrench in one hand and a how-to book in the other, then they may have the technique, but they probably don't have the knack.