Aggregate data is, as the name says, data available only in aggregate form. Typical examples are:

• Turnout for each canton in federal elections: Count (aggregated from individual voters) compared to the overall number of citizens having the right to vote. Note as the basis is the individual, you can aggregate at different levels: voting districts (this is usually the lowest published aggregation level), communes, districts ....
• Unemployment rate: Counts of unemployed and employed persons are compared at some level (communes, cantons, countries,...). Note that the definition of what "unemployed" means varies: it usually does not include persons who have never worked and often also excludes persons who no longer qualify for a unemployment allowance, because the did not have a job for some period...
• Infant mortality rate in countries: Based on counts of children surviving and dying during a specific interval around birth. Note that this calls for a very precise definition; there are national and international definitions that sometimes vary over time. In some countries only children dying in hospitals are recorded; these figures are often estimated and sometimes manipulated (for instance politically decided figures to get some foreign aid). For a particular country the may not have been available for a particular year and the most recently available year has been used (you will of course find this kind of the data in the documentation).

These examples illustrate the importance of getting detailed information on the definition of the measurement and how the data has been collected, and what the problems were.