Let's try to visualize the atoms like pieces of ham or cheese or bread. They still combine in whole number ratios to make molecules, they are just...

 Let's try to visualize the atoms like pieces of ham or cheese or bread.  They still combine in whole number ratios to make molecules, they are just much smaller.For example:

2H2 +  O2   -->   2H2O

This is a 1:2:2 mole ratio.  Therefore if, say 4 moles of hydrogen molecules (H2) were to be available to react, and 6 moles of Oxygen molecules (O2) were available to react, then which reactant would limit the amount of moles of water are produced?   How many moles of water would be produced?  How many moles of the excess reactant would be left over?