Each level requires you to match a certain number of candies with a limited number of moves. At the top of the screen, you can see the icons of the candies you need to collect and each one has a number that shows how many you need. You can see how many moves you have remaining inside the pink jar to the left of the candies.

To collect candies, you need to swap the location of two candies so that three or more of them line up next to each other horizontally or vertically. To swap two candies, left click on the candy and then swipe your mouse in any direction. You can only swipe candy that will successfully result in a match.

Some tiles have special rules. For example, some levels have yellow honey tiles that require you to match on top of them to collect honey. When there are boxes, you have to match candies near the boxes to unlock them. Many levels have unique mechanics like these. You'll have to figure the rest out yourself!