The objective of the game it to turn the color of all the letters to green. To do that you need to find words of three letters or more. The words can be across, down, or diagonal and they can also be in reverse. You mark a word by clicking on its first letter and then on its last letter. The letters that make the word will turn green.
The game would be impossible if not for the fact that once a letter is green you can move it a single space in any direction. When you move a letter it turns back to blue, but it enables you to make new words. You'll get it once you try it. If you're playing for the first time, you should start at the Easy level. Good luck!