The poem "Lesson" by Forrest Hamer is concretely about a boy's trip with his father through Mississippi state. The boy and his father are both afraid of the dangers of the trip, because of racial antagonism and the Klu Klux Klan. Toward the end of the poem, the narrator (the son) tells his audience that his dad will be going to Vietnam soon.
Thematically the poem is about racial tensions. It is illustrated by admitting the year and the state it takes place in. There is obviously the Klan reference. But other lines alert the reader to the fears and dangers. The uses of words like "creatures" and "monsters" are frightening words. The poem also emphasizes darkness several times.
There is also the theme of the importance of family and young boys needing a father figure in their life. "A boy needs a father with him" appears in the last stanza, and if you are looking for a "thesis" of the poem, I think that is it. It comes late in the poem, which usually doesn't occur with thesis statements, but it works in this poem at the end. Knowing that line is a thesis and theme of the poem helps the rest of the poem make more sense. A boy needs his father to teach him about the dangers of the world and to protect him from those dangers as well. The poem focuses on those dangers and then laments that the father will not be around to continue that job, because he must leave and fight in a war far away.