You have the reactants who react to make your products.
Reactants --------> Products
The limiting reactant is the reactant that will RUN OUT FIRST and that will establish the maximum amount of product that will be produced.
To make an example, let's look at this equation and at the following question:
2Al + 6HCl → 2AlCl3 + 3H2
When 6.0 mol Al react with 13 mol HCl, what is the limiting reactant, and how many moles of H2 can be formed?
We know that 2Al + 6HCL are the reactants, right?
These 2 reactants will react to create an amount of a certain product in this case they're asking us for the amount of H2 that will be produced. Therefore, we need to find the REACTANT it can be either AL OR HCL.
But, how do we know which one is the limiting reactant?
To know which one is the limiting reactant we need to know HOW MUCH H2 can Al produce, and also how much H2 can HCL produce. Why?
It's simple, because if you find the one that produces the less amount of H2, you know immediately that's the maximum amount of that product that will be produced.
Let's say you have a sandwich that needs to be made with 2 slices of bread, 3 meats and 1 cheese.
But you have got 4 slices of bread, 9 slices of meat, and 5 slices of cheese.
Well, you could make 2 sandwiches with 4 slices of bread, 3 sandwiches with those 9 slices of meat and 5 sandwiches with the 5 slices of cheese.
But, in reality, you can only make 2 sandwiches because you don't have any more bread to produce more sandwiches. You get it? That is the point of limiting reactants, to find what is the actual amount that can be produced.
Coming back to our equation, we can find the number of moles of H2 produced by each one of the reactants, Al and HCl. I'll find the number of moles quickly to show you what the concept of limiting reactant is.
6.0mol Al x 3 mol H2/2 mol Al = 9 mol H2
13 mol HCL x 3 mol H2/6 mol HCL = 6.5 mol H2
As you can see, 6.5 mol of H2 is the maximum that can be produced by the HCL and is less than the Al, so that maximum amount that you will get in the product H2 is no more than 6.5.
= 8 mol H₂