(… أَأَمِنتُم) (Do you feel secure …): Feeling secure towards anything indicates that this matter does not raise fear or troubles. When we say this person is safe, it means there is nothing he worries about.
(… أَأَمِنتُم مَّن فِي السَّمَاءِ أَن يَخْسِفَ بِكُمُ الْأَرْضَ… ) (…Do you feel secure that He who [holds authority] in the heaven would not cause the earth to swallow you …): In another verse, Allah (swt) says:
أَفَأَمِنتُمْ أَن يَخْسِفَ بِكُمْ جَانِبَ الْبَرِّ أَوْ يُرْسِلَ عَلَيْكُمْ حَاصِبًا ثُمَّ لَا تَجِدُوا لَكُمْ وَكِيلًا (68) أَمْ أَمِنتُمْ أَن يُعِيدَكُمْ فِيهِ تَارَةً أُخْرَىٰ فَيُرْسِلَ عَلَيْكُمْ قَاصِفًا مِّنَ الرِّيحِ فَيُغْرِقَكُم بِمَا كَفَرْتُمْ ۙ ثُمَّ لَا تَجِدُوا لَكُمْ عَلَيْنَا بِهِ تَبِيعًا
(Then do you feel secure that [instead] He will not cause a part of the land to swallow you or send against you a storm of stones? Then you would not find for yourselves an advocate. (69) Or do you feel secure that He will not send you back into the sea another time and send upon you a hurricane of wind and drown you for what you denied? Then you would not find for yourselves against Us an avenger.) (Al-Israa: 68-69)
In this verse, Allah (swt) is warning us that we should think that the land is safe and free of dangers. Certainly not, dangers can be anywhere, whether on land or at sea.
(… أَن يَخْسِفَ بِكُمُ الْأَرْضَ… ) (…cause the earth to swallow you …): Meaning, the earth splits and swallow the objects on it. In this regard, Allah (swt) says in another verse, talking about Qarun:
فَخَسَفْنَا بِهِ وَبِدَارِهِ الْأَرْضَ
(And We caused the earth to swallow him and his home) (Al-Qassas: 81)
Hence, as a punishment the ground splits and swallows what are on it, same as what happened to Qarun when the earth swallowed him and his house which contains his treasures.
( فَإِذَا هِيَ تَمُورُ… ) (…and suddenly it would sway): Who feels secure from being swallowed by the earth while it shakes and sways? Meaning, while the earth turns around and shakes violently until the swallowed things reaches its lower level.