This is a really interesting final image. We've spent the whole poem in a sort of cozy, sleepy homey atmosphere. Now all of a sudden these Arabs creep in at the end of the poem. Longfellow makes them sound like nomads, who "fold their tents" and carry them away with them. We imagine hot deserts under the bright sun and silent men and women in long robes. It's sort of a neat trick, sweeping us off to this exotic world at the last possible moment.
Line 44: The Arabs are part of the final simile in the poem. The speaker is comparing the way your worries disappear at the end of the day to the way that Arab tribesmen pack up their belongings and leave their camp. It's a sort of cool and vivid way of describing the process of unwinding and forgetting about your troubles.