The speaker remembers a hitch-hiker he once picked up, a man who looked like a hobo and carried almost nothing: a small pack and a dog. The speaker hints that this man has been through hard times, and that some strange gift has come out of them. The poem never says exactly what that gift is, but we see it at the end.
After fifty miles of silence, the man asks to be let out because he likes the look of the grass on a hill, and the way the light falls on it. It's a small, odd reason to stop a journey. Most of us would have a destination in mind; he simply responds to beauty and acts on it.
The speaker seems quietly impressed, even a little envious. The tone is gentle and unhurried, with short lines that mimic the slow, plain way the man speaks. The mood is calm and thoughtful, suggesting that having little may leave a person free to notice what others drive past.