With any chemical reaction you have reactants and products, for this question I'll use the example of the formation of water from it's parts, Hydrogen and Oxygen. The equation would look like this:
2H2 (g) + O2 (g) -> 2H2O (l)
Let's say you start out with 3 moles of Hydrogen and 1 mole of Oxygen. From the reaction you can see that for every mole of O2 that reacts, 2 moles of H2 are reacted. This means that after your mole oxygen reacts, you are still left with a mole of excess Hydrogen. Thus, Hydrogen is considered the excess reactant, and Oxygen is considered the limited reactant.
In short, the limited reactant is whichever reactant you will use up in the reaction first.