So the lower your number, the slower your shutter speed. This means you'll be able to get more light into your camera BUT slow shutter speeds can cause blur. When your shutter is open it is recording the image. If your shutter speed is open for too long it will register movement which will cause a blurry photo. Let's face it, you can't hold the camera perfectly still and when shooting people, they often can't hold perfectly still either.
If your subject is holding still and you hold your camera as steady as you can, many photographers are able to shoot at a shutter speed of 30. I'll be honest, I don't have the steadiest of hands so I prefer to be at at least 50. A good way around this problem is to use a tripod. You can get away with much lower shutter speeds when using a tripod.
Here's what happens when you shoot with a slow shutter speed:
I was outside and starting to lose sunlight. My shutter speed was around 20, maybe even a little slower. I had my ISO very low so all I needed to do in this case was crank up the ISO from 200 to 600 or so and I would have been able to raise my shutter speed.
Now if you are shooting a moving subject (ie. children) your shutter speed needs to be in a whole different range. With a moving subject it's best to be around 160 or higher. Granted, most children aren't always moving so you could probably shoot around 100 or 120 and get a majority of crisp images (depending on the child of course). But if your subject is really moving you're going to want to be around 200 or higher.
SIDE NOTE: Keep in mind that if your subject is moving towards or away from you (rather than across your screen) you can run into focus issues that can also cause blur. That's where continuous focus can be helpful. Here's a good explanation of the different focus modes on Nikons. It is different for Canon users so if you shoot with a Canon you may want to run a google search on focus modes for Canon. The article I linked gives a great explanation of the modes, but you may want to search on youtube or google for how to change the setting on your particular camera.
See how crisp and sharp this little guy's eyes are? His eyes were my focus point (as should always be the case when shooting people). You'll notice his little body and the blanket become blurry but that is because I am shooting with an aperture setting of 1.4 and am very close to my subject. I'm hoping you remember from our last lesson that that will give me a very shallow depth of field. (ie. his face is in sharp focus but things on a different plane become quite blurry).
Shutter speed is usually my most variable setting. It's the one that I usually change through a session as I need. However, I keep an eye on it. If it starts to creep down past 50 (higher if my subjects are really on the move) I increase my ISO enough that I can raise my shutter speed. I don't like cranking my ISO up too high when I'm doing a professional shoot BUT it is much better to have a grainy image than a blurry one, so sometimes you've got to do it. However, if you ever find yourself having to go above ISO 800 for a print that the family may want enlarged (or much above 1600 in any case) you should probably invest in a better lens that will allow you to open your aperture wider, getting more light and allowing you to lower your ISO and/or raise your shutter speed.
I hope all of that made sense. Our next lesson will be how to actually put it all together and actually shoot in manual mode! Comment here with any questions!
Homework: I bet you've guessed it already! Head on over to youtube and find out how to change your shutter speed. You don't have to take any photos yet (although you're more than welcome to if you're feeling brave) but I do want you to practice changing all of the three settings we've gone over.