Can you explain what an index without a specific order in a database is?

A heap index is a data structure in a database that does not support key sorting. Such an index simply stores references to table rows without ordering by key value. Unlike ordered index structures like B-trees, a heap index does not speed up range searches or sorting, but can be used for quick access to data by exact match or for storing data without the need to maintain order. An example could be a heap table in some DBMSs, where data is stored without a specific order.