thats how long the shutter stays open during a shot. if you are getting 1.5", 1" means the shutter opens for 1.5sec and 1sec respectively. when you see just 10 without the "(10)
it means shutter opens for 1/10 seconds... think of light as water, aperture as opening, and ISO as flow rate.
say your setting is ISO200 at f2.8, 1/5s....so in order to decrease the aperture opening time, you can either open the aperture wider or increase the flow rate. since 2.8 is already the widest opening, that leaves you the choice of increasing the ISO. from 200 to 400 you have doubled the ISO, so now the shutter will open for 1/2 the time of 1/5 which gives you 1/10 seconds.
think of filling a pail w water considering ISO(flow rate), aperture( f2.8, f4 etc) and shutter speed( how long to saty open)...changing either of this will affect the other....