how to choose inner tubes
700 is the size of the wheel and tire which the tube is intended to fit. In this case, 700 means 700c, or ISO diameter 622mm rims. If you are unfamiliar with tire sizes, take a look here.

28/32 designates the min/max range of width of the tire which the tube is intended to fit, in mm. So in your case, your tubes are designed to fit tires of width 28mm (minimum) to 32mm (maximum). This range should correspond to the width of your rim as well.

Edit: From @Benson's comment on this answer (in case it gets deleted)


  Using a tube that's too big will mean it's exceedingly hard to get your tire back on, because the tube will have to fold out of the way. You'll risk pinching and damaging the tube as you put your tire back on. If you use a tube that's too small, you'll risk the tube stretching too far, which makes it much more vulnerable to popping.