In the F/x number the F stands for Focal Lenght. Let's take the kitlens at 18mm for example:
- at F/3.5 the diaphragm is 18/3.5 = 5.14mm
- at F/8 the diaphragm is 18/8 = 2.25mm
So the larger the F-number, the smaller the diaphragm is.
See for yourself why you should stop it down a little for better results:
Oh, and I'd like to add: you might consider picking up a EF 50 1.8. It's only €80 or so, tacksharp from F/2.8 on and great to play with a small DOF @ F/1.8 to let those details stand out on cars for example.