In a chemical reaction, what is the mass of the product ?
The limiting reactant is what gets used up completely in the reaction, while the other reactant(s) in excess will remain by the time the reaction is over.
Let's consider an example where we have two reactants
#A + B -> C#
#"Mass of C" = cancel"g A" xx ("1" cancel("mol A"))/cancel"g A" xx "mol C"/cancel("mol A") xx "g C"/cancel"mol C"#
For a more explicit example, recall the components of a combustion reaction (hydrocarbon + gaseous oxygen yields gaseous carbon dioxide and gaseous water).
In many cases, oxygen is in excess and the hydrocarbon being combusted is the limiting reactant.