Stoichiometry is the study of ratios of elements or compounds and the way they react with each other in specific ratios or quantities.  Oftentimes, moles are used in describing these releationships between elements and/or compounds.  Moles are useful because they describe the number of a specific element required to react with a specific number of another element.  Consider the formation of water:

2H2  +  O2  --->  2H20

There are 2 moles of hydrogen required to react with one mole of oxygen which will produce 2 moles of water.  So you would need twice as much hydrogen as oxygen to produce water.  Another way moles come into play is describing the quantitative relationhips of the structure of compounds.  Consider a mole of glucose, C6H12O6.  What this means is there are 6 moles of carbon, 12 moles of hydrogen, and 6 moles of oxygen required to make one mole of glucose.

Moles means avogadro number of molecules or atoms etc.,

Moles of a molecule = mass/ molar mass

Moles of a atom = mass/ atomic mass.

in Stoichometry moles is used at various places like 

finding mass.

mass = moles * molar mass

Finidng moles is the first step in finding limiting reactant.

Example

find the moles of 10 grams of Sodium hydroxide (NaOH)

moles = 10/40      [ Molar mass of NaOH = 40]

         = 0.25

Example

find the mass of 0.5 moles of calcium carbonate

moles = mass / molar mass

0.5 = mass/ 100   [ molar mass of Calcium carbonate = 100]

mass = 0.5 * 100

         = 50 grams.

Finding moles is also used at different stages, depending upon the problem need