وَجَعَلْنَا فِي الاَْرْضِ رَوَاسِيَ
...
And We have placed on the earth firm mountains,
means, mountains which stabilize the earth and keep it steady and lend it weight, lest it should shake with the people, i.e., move and tremble so that they would not be able to stand firm on it -- because it is covered with water, apart from one-quarter of its surface. So the land is exposed to the air and sun, so that its people may see the sky with its dazzling signs and evidence.
So Allah says,
...
أَن تَمِيدَ بِهِمْ
...
lest it should shake with them,
meaning, so that it will not shake with them.
...
وَجَعَلْنَا فِيهَا فِجَاجًا سُبُلً
...
and We placed therein broad highways for them to pass through,
means, mountain passes through which they may travel from region to region, country to country. As we can see, the mountains form barriers between one land and another, so Allah created gaps -- passes -- in the mountains so that people may travel from here to there.
So He says:
...
لَعَلَّهُمْ يَهْتَدُونَ
that they may be guided.