For example;
If you have a 5 x 7 matrix of LEDs then you'd have 5 rows and 7 columns.
Now then, to light a particular LED (I.E. top left) you'd need to apply power to row 1 and ground column 1.
OK, so if you wanted another LED lit on row 1 you ground the column that it is in. Next, you switch power from row 1 to row 2 and ground the columns that you want lit.
Do this for all rows one after the other really quick and you can have whatever you want on the display. You have to do it quick because then your eyes will see a complete image even though only one row is lit at any one time.