As per Wikipedia, the term denotes the "eternal" or absolute set of duties or religiously ordained practices incumbent upon all Hindus, regardless of class, caste, or sect.
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Sanātana Dharma (सनातनधर्म) was first mentioned in Rig Veda. It indicates the eternal rules to be followed by a spiritual practitioner, but nothing to do with any religious duties.
The rishis of Rig Veda, who composed the Riks, in general, convey their ideas on spirituality in an esoteric sense only, but not in a direct manner.
वैश्वानराय पर्थुपाजसे विपो रत्ना विधन्त धरुणेषु गातवे |
अग्निर्हि देवानम्र्तो दुवस्यत्यथा धर्माणि सनता न दूदुषत् ||
“Agni, Vaiśvānara of broad visage and friend of Gods, will support strongly and bestow treasures (wisdom/SELF REALISATION), on the spiritual seeker, who will not deviate from the everlasting laws”.
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We have to remember that the original Vedic Sanskrit word for of old/eternal is sanatā - सनता, but not sanātana, which is a word in Classical Sanskrit - systematized by Pāṇini .
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Whether there is hell or heaven in Sanatana Dharma?
If we strictly go with Rig Veda, then there is no hell or heaven.
(i) Concept of Immortality
We have in Rig Veda the concept of attaining Amṛtatva (अमृतत्व) - Immortality.
Immortality does not indicate living for eternal period on the Earth. It indicates liberation or moksha.
त्र्यम्बकं यजामहे सुगन्धिं पुष्टिवर्धनम् । उर्वारुकमिव बन्धनान्मृत्योर्मुक्षीय मामृतात् ॥
tryambakaṃ yajāmahe sugandhim puṣṭivardhanam | urvārukam iva bandhanān mṛtyor
mukṣīya māmṛtāt ||
English translation:
May we adore/meditate the ONE with 3 mothers (tryambakaṃ - त्र्यम्बकं), whose DIVINE ILLUMINATION (sugandhim - सुगन्धिं) pervades everywhere and who strengthens our meditation/prayers (puṣṭivardhanam - पुष्टिवर्धनम् ). Like a cucumber from its stem, (might I) be freed (mukṣīya - मुक्षीय) from death, not from deathlessness (Amṛtatva).
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(ii) Undergoing the results of past karmas.
Human being is expected to get rid of his weaknesses/the result of past karmas, including that of one’s ancestors, on the Earth itself, in order to progress in spiritual realm.
अव द्रुग्धानि पित्र्या सृजा नोऽव या वयं चकृमा तनूभिः । अव राजन्पशुतृपं न तायुं सृजा वत्सं न दाम्नो वसिष्ठम् ॥
ava drugdhāni pitryā sṛjā no 'va yā vayaṃ cakṛmā tanūbhiḥ | ava rājan paśutṛpaṃ
na tāyuṃ sṛjā vatsaṃ na dāmno vasiṣṭham ||
English translation:
Release from us ancestral deceits and those that we ourselves have committed. O king, release Vasiṣṭha from his bond like a cattle-stealing thief, like a calf.
