If a spreadsheet table could be classified as a database, it would be known as a "flat file" database. It stands alone and is related only to itself. Picture a spreadsheet listing customers, their names and addresses, the products they purchased and the price they paid. Take that same concept to a database, but instead of one table, we have a table for customers names and address, one for products and their costs, one that keeps track of inventory, and a table that brings together information from the latter to record the individual transactions of a company's daily business.
The idea behind maintaining separate tables is that it is more efficient. Say you have a change in price or supplier for a product. Make that change in the products table, and all future transactions involving that product will automatically reflect the new information. That information is all brought together through table relationships that match one field (row) in each table to a like field in a related table. For example, in a customer's table, you might have a field called "Product Code." That field would be joined with your products table which would have the same field. You join the tables in the database, and like magic you can keep everything joined, but manageable.