It's pretty much a fancy way of saying a total amount. Thefreedictionary.com defines aggreates as;
"1. Constituting or amounting to a whole; total: aggregate sales in that market.
2. Botany Crowded or massed into a dense cluster.
3. Composed of a mixture of minerals separable by mechanical means.
1. A total considered with reference to its constituent parts; a gross amount: "An empire is the aggregate of many states under one common head" (Edmund Burke).
2. The mineral materials, such as sand or stone, used in making concrete."