What does verse 13:31 of the Quran mean?

The Quran is written in a poetic language and some verses are not direct. But just as poetry, you can understand the meaning from the context. This verse is such a verse and it translates as follows in English:

"And if there was any Qur'an by which the mountains would be removed or the earth would be broken apart or the dead would be made to speak, [it would be this Qur'an], but to Allah belongs the affair entirely. Then have those who believed not accepted that had Allah willed, He would have guided the people, all of them? And those who disbelieve do not cease to be struck, for what they have done, by calamity - or it will descend near their home - until there comes the promise of Allah. Indeed, Allah does not fail in [His] promise."

This verse emphasizes the power and authority of Allah, stating that if there were a scripture capable of miraculous feats such as moving mountains, splitting the earth, or making the dead speak, it would be the Quran. However, it asserts that Allah alone holds absolute control over all affairs, and his will determines guidance and outcomes. It also warns those who disbelieve that they will face consequences for their actions until the fulfilment of Allah's promise.

The first sentence of the verse abruptly finishes in the Arabic Quran. I have put “……” there to indicate the gap, but English translators filled this gap according to their understanding. The probable word in this gap here according to the context is “they will not still believe”. That means if the quran was a magic mantra and it could move a mountain, the Meccans would still not believe. That’s why the next verse say God can make them forcefully believe but that will be against free will (also stated in 10:99: “Had your Lord so willed ˹O Prophet˺, all ˹people˺ on earth would have certainly believed, every single one of them! Would you then force people to become believers? “). The below verse has the same concept of your verse.

17:90 and so they say, "We will not believe you [Prophet] until you cause a spring to gush up from the earth; or,

17:91 unless you have a garden of dates and grapes and cause rivers to pour through it abundantly.

17:92 Or, until the skies fall down upon us-crushing us, as you have threatened to do. Or [until] you bring God and the angels face to face before us,

17:93 or, to have a house of gold or you go to heaven-but we would not even believe in your ascension unless you return with a book that we can read! "

So, if the Quran could move mountain, the Meccans would still not believe.