In this example, you can see that the entry date is not in date order. Neither are the entries in columns C, D and E. By contrast, the Income column is determining the order. It goes from low to highest. This is the index field. It may be that another determinant could be chosen for the Index field, such as number sold. The essence is that the index field is the area of interest; i.e. what the data output is to show.
Autonumber field only helps with the order in which they were created. Helps determine if something was deleted and how many records you have. However,the order of columns in a multi-column index definitely matters. One type of query may need a certain column order for the index. If you have several types of queries, you might need several indexes to help them, with columns in different orders.