Most organizations store their data in data stores such as databases, data warehouses, or data lakes. The flow of information can come from different systems or tools. Most of the time, the data is stored in a relational database composed of multiple tables rather than a single one with well-defined relationships between them.
For instance, an online store could have multiple tables for recording all the purchases made on its platform. One table might contain information relating to existing customers, another one might list all existing and past products in the catalog, and a third one might contain all of the transactions that occurred, and so on.
If you were working on a project recommending products to customers for an e-commerce platform such as Amazon, you may have been given only the data from the transactions table. In that case, you would like to get some attributes for each product and customer and would have to ask to extract these additional tables...