What does this mean?  It means that you have to adjust the exposure time to a value relative to your lens and your own abilities.  The "golden rule" is as follows.  Exposure length is to be the inverse of the focal length.  In other words, if you use a 50mm lens, you can hold 1/50s freehand (most cameras will have 1/60s as their neares setting).  Thus, with a 200mm lens, you need a speed of 1/200s, or with a 28mm tele, you can take a picture at 1/30s.  That's the guideline.  Sharpness fanatics will tell you that you have to decrease the exposure time by a factor of four. For a 50mm lens, they will recommend 1/250s (not 1/60s).  Available light fanatics will tell you that you can go down by a factor of two, as long as you are careful, have a good stand, and use proper technique.  That way, you could shoot a 50mm lens at 1/15s. 

I usually stick with the golden rule.  When the light gets low, however, I will shoot my 200mm lens at 1/125s, or if  I really want the picture, I might even try 1/60s.  But I will then try to control my breathing, "ooze" the shutter release button down (not press it hard), or stabilize the camera by leaning against something stable.  I may also take two pictures instead of one, hoping that I may be able to hold at least one steady. 

Another not-photographer related problem with long exposures is the movement of the object.  People may not stand still for 1/15s, so your picture will show heads that are turning or arms in motion with long streaks.   This can be interesting, but frequently just ruins the shot. 

(c) Christian Wagner, 2001.

Picture taken without flash, about 1/8s. Lots of motion blurr.

Same picture with flash, at 1/60s.  The shorter exposure time freezes the action.