Each number 1-9 must appear once and only once in each row, column and box.
Complete the grid by finding and filling in cells where one (and only one) of the numbers (1-9) hasn't already been used in that corresponding row, column and box.
In the process of eliminating available slots as you place more of the (one and only) correct numbers to the cell in which they belong, you are providing more clues for the remaining cells.
Start with puzzles that are indicated as EASY, where there is only one possible and obvious choice. Once you grasp the concept you will be ready to employ more complex strategies in locating and determining the only possible correct number to place in a particular cell.
Here are some easy 4x4 grids that might help you to grasp the concept.
http://1sudoku.com/play/sudoku-kids-free/sudoku-4x4/