The intent by the "edge" (al-Harf) here is the deviation (al-Inhiraf) from correctness, which is the Truth, according to the saying of the Apostle Paul in his epistle to Titus:

"As for a person who stirs up division, after warning him once and then twice, have nothing more to do with him; knowing that such a person has deviated and is sinful, being self-condemned." (Titus 3:10-11).

And in the interpretation of al-Jalalayn: The edge (al-Harf), meaning doubt in his worship; he is likened to a state on the edge of a mountain in his lack of stability. And "good" is: health and safety in himself and his wealth; and trial (al-fitnah) is the ordeal and sickness in his self and wealth. "Turns" means he returns to disbelief (al-kufr). He loses this world by missing out on what he hoped from it, and the Hereafter by disbelief. That is because he invokes through his worship besides God that which neither harms nor benefits him of the worldly exoteric named things (al-mussamayat al-zahirah), and believing in them is the far-off error from the truth. And He Almighty said in Surat al-A'raf, refuting their interpretation: {Do they wait for anything except its fulfillment [ta’wiluh]?} (Al-A'raf: 53).

And He Almighty informed us that the people of deviation follow nothing but the ambiguous verses by His saying: {It is He who has sent down to you the Book; in it are verses [that are] firm (muhkamat) - they are the foundation of the Book - and others unspecific (mutashabihat).