Flat data resides in a single table.
In contrast to a relational data structure, a flat data structure is a simple list that isn't related to other data. For example, a grocery list is a flat file, and so is a list of your friends and relatives. Flat data structures are easy to create, and they're also easy to maintain, as long as you don't have too much information.
Simple lists work nicely in Excel. In fact, Excel is designed to create and maintain flat files.
In case you're wondering, each table in a relational structure is also a flat file. For example, in a typical sales database, one table holds a list of customers, and the other holds a list of orders. The only real difference between the types of structures is that with flat files, you don't need to relate the data in one list to the data in another. Each list is useful by itself.