"Leaving money on the table" is an idiom which means not getting as much money as you could.
You use this phrase to talk about negotiations, finance, and buying and selling things. For example:
If you're going to college and you don't apply for any grants or scholarships, you're probably leaving money on the table.
The "table" in this expression comes from poker: when you don't play intelligently, you're leaving your money on the poker table.