This is the default query mode: the whole query result is evaluated upon query execution and an object IDs list is produced as a result.
Obviously evaluation takes some time and in a case of big result sets you will have to wait for a long time before the first result will be returned. This is especially unpleasant in a client-server setup, when query processing can block the server for seconds.
This mode makes the whole objects result set available at once. An result list is built based on the committed state in the database. As soon as a result is delivered it won't be changed neither by changes in current transaction neither by committed changes from another transactions.
Note that the result set contains only references to objects you were querying for, which means that if an object field has changed by the time of the actual object retrieval from the object set - you will get the new field value: