The limiting reagent is the reactant which runs out first. After it runs out, the reactant cannot proceed further. It "limits" the reaction. All other reactants are in "excess".
When calculating how much product can be made, always start calculations with the limiting reagent because it determines how much product can be made.
This is an analogy which helped me:
Suppose we are making sandwiches. For each sandwich, we need 2 slices of bread, 3 tablespoons of peanut butter, and 1 tablespoon of jam.
If we have the ingredients in that exact ratio, we can make sandwiches with no leftover "reactants".
Suppose we only have 4 slices of bread. You could have all the peanut butter and jam in the world, but you can only make 2 sandwiches because there are only 4 slices. In this case, the bread would be the limiting reagent.
The limiting reagent is not always the one with the least amount (grams or liters). It is based on mole ratios from the balanced reaction equation.
Note: limiting reagent is also sometimes referred to as limiting reactant.