Aggregation is just the like English word meaning - a cluster of things that have come or been brought together.

It brings similar types into one group Ex: columns, rows in a table or content in page.

For more, go through documentation in https://sapui5.hana.ondemand.com

please provide more context.. what are you trying to achieve? what does your code look like?

an aggregation is similar to a list, rows, etc.. something that can be populated to display a data set, such as a list in a drop down box. or items(rows) on a table