The needy traveller, serene and gay, Walks the wild heath, and sings his toil away. Does envy seize thee? Crush th' upbraiding joy, Increase his riches, and his peace destroy-- Now fears in dire vicissitude invade, The rustling brake alarms, and quivering shade; Nor light nor darkness brings his pain relief, One shows the plunder, and one hides the thief.
A poor traveller, according to the speaker, is happy and at peace. He walks through the wilderness singing all his troubles away. But the minute envy seizes him (note the personification there), he is no longer happy. If his wealth increases, his peace is destroyed.
Rustling leaves alarm him, and the shade scares him. Neither light nor darkness brings him relief from his pain and his troubles. Light shows him what's been stolen from him, and darkness hides the thief who has stolen from him. In a nutshell: it's bad times, indeed.