Tafsir of Al-Kahf 18:92

{Between the two barriers} Between the two mountains. They are two mountains that Dhu al-Qarnayn blocked the space between.

Regarding the reading of "Sadd" (barrier): It is read with both a damma (Sudd) and a fatha (Sadd). It is said that whatever is of Allah’s creation is read with a damma, and whatever is of human labor is read with a fatha. This is because al-Sudd (with damma) is a noun acting as a passive participle, meaning: it is something Allah created and fashioned. Al-Sadd (with fatha) is a verbal noun for an event performed by people.

Grammatical note on "Bayna" (between): It is in the accusative case (nasb) as a direct object (the location reached), just as it is in the genitive case (jarr) by annexation in the verse: "This is the parting between me and you" (18:78), and in the nominative case (raf') in the verse: "The ties between you have been severed" (6:94). This is because it is among the nouns that function both as adverbs of place and as nouns. This location is at the edge of the land of the Turks, towards the East.

{Besides them, a people} They are the Turks.

{Who could hardly understand speech} They could hardly understand it except with great effort and difficulty, through gestures and the like, as one would make someone understand you. It is also read as yufqihun (to make others understand), meaning: they do not make the listener understand their speech, nor do they clarify it, because their language is strange and unknown.