It's a method whereby we convert one unit into another. For example, suppose we want to convert 15 inches to feet. We know there are 12 inches/foot so we set it up like this.
15 inches x (1 foot/12 inches) = ??feet. You can see that inches in the numerator cancel with inches in the denominator. So the units we DON'T want to keep cancel. The unit we want to convert things to stay. So 15 inches = 1.25 feet.
You can use this to convert one unit to another (any system), to convert chemical equations from moles of one reagent to another, or to convert rates of reaction on one material to rates of reaction of another. This is a general discussion. Try it yourself; if you get confused post a real problem and tell us what you don't understand about it.
(Note: The above is a working definition of how it is used; another meaning is to set up a problem in chemistry, physics, math, what have you, and use just dimensions with no numbers. Then you see if the units for the problem come out in the units you want. If so it is good evidence that you have set up the problem correctly. For example, if we have a problem to calculate the energy in joules of light of wavelength 500 nanometers, we can set up just the units.
E = hc/wavelength.
h is Planck's constant in Joule*sec = J*s, c is speed of light in meters/sec= m/s and wavelength is in meters =m.
So E = (J*s*m/s)/m.
s cancels, m cancels, to leave J and that is what E is measured in so we can be relatively certain this is the way to attack the problem.) Hope this helps.