The Reward of those Who believe and do Righteous Deeds
When Allah mentions the state of those who are doomed, He follows that by mentioning the blessed who believed in Allah and believed what His Messengers brought, those who did the righteous deeds that they commanded them to do. They will have Jannatu `Adn. `Adn means lasting.
تَجْرِى مِن تَحْتِهِمُ الاٌّنْهَـرُ
(wherein rivers flow beneath them,) means, from beneath its rooms and dwellings. Fir`awn said:
وَهَـذِهِ الاٌّنْهَـرُ تَجْرِى مِن تَحْتِى
(and these rivers flowing beneath me...) 43:51
يُحَلَّوْنَ
(they will be adorned) means, with jewelry.
فِيهَا مِنْ أَسَاوِرَ مِن ذَهَبٍ
(with bracelets of gold,) Allah says elsewhere:
وَلُؤْلُؤاً وَلِبَاسُهُمْ فِيهَا حَرِيرٌ
(and pearls and their garments therein will be of silk) 22:23. This is explained in more detail here, where Allah says:
وَيَلْبَسُونَ ثِيَابًا خُضْرًا مِّن سُنْدُسٍ وَإِسْتَبْرَقٍ
(and they will wear green garments of Sundus and Istabraq.) Sundus refers to a fine garment, like a shirt and the like, and Istabraq is thick and shiny velvet.
مُّتَّكِئِينَ فِيهَا عَلَى الاٌّرَائِكِ
(They will be Muttaki'in therein on Ara'ik.) The word Muttaki'in implies lying down, or it was said that it means sitting with one's legs crossed, which is closer to the meaning here. In a Sahih Hadith, the Prophet said:
«أَمَّا أَنَا فَلَا آكُلُ مُتَّكِئًا»
(As for me, I do not eat sitting with legs crossed (Muttaki'an)). Ara'ik is the plural of Arikah, which is a bed under a canopy. And Allah knows best.
نِعْمَ الثَّوَابُ وَحَسُنَتْ مُرْتَفَقاً
(How good is the reward, and what an excellent place of rest (Murtafaq)!) means, how blessed is Paradise as a reward for their good deeds. And what an excellent Murtafaq means, and how good a place to dwell and rest and stay. Previously, Allah had said of Hell,
بِئْسَ الشَّرَابُ وَسَآءَتْ مُرْتَفَقًا
(Terrible is the drink, and an evil place of rest (Murtafaq)!) 18:29. In a similar way, He contrasts the two (Paradise and Hell) in Surat Al-Furqan, where He says:
إِنَّهَا سَآءَتْ مُسْتَقَرّاً وَمُقَاماً
(Evil indeed it (Hell) is as an abode, and as a place to rest in.) 25:66. Then He mentions the qualities of the believers, then says:
أُوْلَـئِكَ يُجْزَوْنَ الْغُرْفَةَ بِمَا صَبَرُواْ وَيُلَقَّوْنَ فِيهَا تَحِيَّةً وَسَلَـماً - خَـلِدِينَ فِيهَا حَسُنَتْ مُسْتَقَرّاً وَمُقَاماً
(Those will be rewarded with the highest place because of their patience. Therein they shall be met with greetings and the word of peace and respect. Abiding therein excellent it is as an abode, and as a place to rest in.) 25:75-76
Tafsir Ibn Kathir (abridged, English).