Allah states that even though the martyrs were killed in this life, their souls are alive and receiving provisions in the Dwelling of Everlasting Life. In his Sahih, Muslim recorded that Masruq said, "We asked `Abdullah about this Ayah,
So, in summation, their first merit as pointed out in this verse is their distinct perennial life; the second is their being well-provided from Allah and the third (فَرِحِينَ بِمَا آتَاهُمُ اللَّـهُ ) is that they shall always be happy with Allah's blessings and grace. The fourth (وَيَسْتَبْشِرُونَ بِالَّذِينَ لَمْ يَلْحَقُوا بِهِم) merit is the good news given to them about their relatives and circle of friends they left behind in the mortal world that they too shall deserve the same blessings and ranks with their Lord if they remain good in deeds and are active in Jihad (170-171).
Al-Suddi says that a شھید shahld is informed beforehand when one of his close friend or relative is about to die. The news that a certain person was coming to them makes them as happy as one would usually be when an old friend, long separated by time and distance, comes to meet him.
Those who participate in the encounter of truth and untruth in such a way as to make real sacrifices for the sake of truth, are scorned by worldly people as having destroyed themselves for no good reason. But this is a foolish assessment. Incurring a seeming loss in God’s path is a positive happening, because those who have sacrificed everything for God are the ones most deserving of God’s rewards. The foolish speak of those who sacrifice their lives in the path of God as if it is only they who die, as if those who are striving towards worldly ends do not taste death. This is entirely meaningless and those who speak of such things are never serious. Their hearts tell them that they have made a great mistake in not making sacrifices for the cause of truth, but by speaking ill of those who have done so, they want to keep up a pretence that they are in the right. But death is decreed for all human beings by God. It will certainly visit everyone at the appointed time. Whichever path one follows, none can escape from the clutches of death. People realize their mistake in their hearts, but they do not want to acknowledge it. In order to justify their course of action, they utter such words as even their own hearts attest to as being false.
Uthmani text of the Madinah mushaf (King Fahd Complex). Translation: Saheeh International. Tafsir via Quran.com.