Aggregate tables store pre-computed results, which are measures that have been aggregated (typically summed) over a set of dimensional attributes. Using aggregate tables is a very popular technique for speeding up query response times in decision support systems.
Ex : If you observe that most of the queries are fired on aggregated values like yearly or monthly data rather that day level data then to speed up the report we can create an aggregate table containing month level and year level data and map it to the respective logical table and set the aggregation levels in the content tab of LTS.