Development tiles are symbols which are added to the Basic Action shown on the tile. You need them to increase the value of that action every time that you perform it.
When you take a Development tile, place it in one of the free drawer spaces in the second row of your desk. If all 5 of the drawers in your desk are already full, you can replace one of the Development tiles you placed earlier in the game.
There are 20 development tiles used in the game.