Consider a combustion reaction (of say methane):
#CH_4(g) + 2O_2(g) rarr CO_2(g) + 2H_2O(g)#
There is lots of oxygen in the atmosphere; there is limited methane in your gas bottle. Clearly, here methane is the limiting reagent.
Many problems require you to identify the limiting reagent. If you have a stoichiometric equation, most of the time the limiting reagent is easily designated.