This phrase is the translation of the Latin habendum et tenendum that historically commenced these clauses in deeds.
The obsolescence of land tenure, however, renders this distinction mostly historical and academic.
For example, if the grantor conveys a time share interest or an interest less than fee simple absolute, the habendum clause would specify the owner's rights as well as how those rights are limited (a specific time frame or certain prohibited activities, for instance).
The habendum clause can define how long the interest granted will extend.
The secondary term is usually "so long thereafter as oil and gas is produced in paying quantities."