In a database, there are rules about where different entities are stored. For instance, everything about the customers should be stored in the Customers table. A customer identifier is stored in this table, which means that the necessary data can be retrieved by a simple lookup in the Customers table. So, if you need to refer to a customer from another table, you just store this identifier in the other table.
The identifier needs to be unique in the Customers table, so that every record can be addressed. Here the field is called a primary key. In other tables, duplicates may exist. For example, several records in the Orders table might refer to the same customer. If this is the case, the key is called a foreign key.
A consequence of this setup is that a database can have many tables, often thousands. One table for customers, one for orders, one for order lines, one for products, one for product groups, one for shippers, one for invoices, and so on. Each table...