Hmm well I'll give it a go!
If you want the model in focus and the background blurry then you need to have a shallow or wide depth of field. This means that you are choosing a low deptyh of feeling (a low number) for example. f/8. The smaller the number the greater the effect will be.
Just remember that the lower the number the wider the lens will be open. The more open it is the less in focus the background will be.
It makes the shutter speed faster technically - because you are letting more light in as the lens is more open.