"The descent to the infernal regions is easy enough, but to retrace one's steps, and reach the air above, there's the rub"

This quote by Virgil is an allegory for the problem of overcoming hardship. It recommends that it is easy to fall under a difficult situation, however it is much more difficult to get out of it. The "descent to the infernal regions" is a referral to the descent into a tight spot, while the "air over" is a recommendation to the relief of conquering the hardship. The phrase "there's snag" suggests that the trouble hinges on the effort called for to overcome the adversity.

This quote is a suggestion that it is easy to fall under difficult situations, but it takes a lot of effort and also dedication to leave them. It is a pointer to stay strong and keep pressing ahead, even when the going gets tough. It is likewise a suggestion that it is possible to conquer difficulty, regardless of just how tough it might appear. With sufficient effort as well as resolution, it is possible to get to the "air over" and discover relief from the tight spot.