In the opening line of the poem, the speaker describes a "Stranger within my gate." This is an example of imagery, and the image creates the impression that the stranger is an intruder, trespassing upon the speaker's land, and approaching the speaker's home. This image sets up the rest of the poem, in which the stranger is represented as a threat to the speaker.
The eponymous stranger in the poem is a personification of all those people who the speaker considers foreign or different. The speaker is hostile to people from different cultures because he "cannot tell what powers control," or "What reasons sway (their) mood." He prefers "men of (his) own stock," who, even if they should be "Bitter bad," at least speak the same language as him, and believe in the same God as he does.
In the final stanza of the poem, the speaker declares, "Let the corn be all one sheaf - / And the grapes be all one vine." This is a metaphor whereby the corn and the grapes represent people, and the sheaf and the vine represent either race or culture. The speaker is proclaiming that, in his opinion, people of different races and cultures should not mix with one another.