کنیهٔ اباصالح برای امام زمان (عج) به چه معناست؟

An answer that may seem intuitive or merely a personal opinion is, in fact, the absolute truth:

The Imam of our time, peace be upon him, is the father of those who will inherit the earth. The righteous will inherit the earth, and our Imam, peace be upon him, is their leader and father. The Imam of our time, peace be upon him, is the father of the Shi‘a; in a word, to be Shi‘a is to be righteous. Indeed, apart from the Prophet Muhammad, peace be upon him, Imam Mahdi, peace be upon him, stands as a father to the prophets!

الأنبياء : 105 وَ لَقَدْ كَتَبْنا فِي الزَّبُورِ مِنْ بَعْدِ الذِّكْرِ أَنَّ الْأَرْضَ يَرِثُها عِبادِيَ الصَّالِحُونَ
In the “Psalms,” after the mention of the Torah, We wrote: “My righteous servants shall inherit the earth!”

في مَجْمَعِ الْبَيَانِ «أَنَّ الْأَرْضَ يَرِثُها عِبادِيَ الصَّالِحُونَ» و قَالَ أَبُو جَعْفَرٍ عَلَيْهِ السَّلَامُ: هُمْ أَصْحَابُ الْمَهْدِيِّ فِي آخِرِ الزَّمَان‏
Tafsīr Nūr al-Thaqalayn, vol. 3, p. 464

The companions of Imam Mahdi, peace be upon him, at the end of time are those same righteous servants of God who will inherit the earth.