Until now you asked Elasticsearch "give me the documents that…". Aggregations change the question: "how many courses per category? what average price? how many accesses per day? what latency for 99% of requests?". It is SQL's GROUP BY, but more flexible: you can nest groups (category → level), place a metric in each group (average price per category), and even bring back the best documents of each group. All in the same _search request, alongside a classic search or in its place. This is what Kibana does under the hood for every chart you will see in module 4.
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