Generally speaking, aggregation refers to the process of bringing things together. The term has become common in a lot of fields. In media, there is news aggregation, which brings together content from a number of different sources. In technology, there is data aggregation, which brings together a number of different data sets. (In our stomachs, there's burrito aggregation, but that's a story for a different time.)
In finance, the term often refers to the combination of various investments into a single accounting document. Investors, especially of the top-hat-wearing, monocle-polishing, money-bin-building type you see so often in line at Starbucks these days, often have diverse and complicated financial entanglements. This diversified portfolio can make it hard to review all of person's holdings at once. By aggregating the information, financial professionals (and the investors themselves) have a better understanding of where they stand and where their ongoing risks and opportunities lie.
