In "Because I could not stop for Death--" by Emily Dickinson, what is the difference between the word "stop" in line 1 and line 2?

Grammatically, there's a small difference between the two appearances of that verb in the first stanza, but the main difference is in meaning and tone:

Because I could not stop for Death –

He kindly stopped for me –

The Carriage held but just Ourselves –

And Immortality.

The "stop" in the first line appears with no verb conjugation because of its placement near the auxiliary verb "could." And the "stop" in the second line includes a past tense "-ed" ending because it's simply attributed to the subject, "He," without any intervening auxiliary verbs. It's like the difference between saying "I didn't do my homework" compared to "He always did his homework."

More to the point, though, in the first line, the speaker means that she couldn't slow down her activities, or that she couldn't set aside the business of living, in order to allow her own death to approach. In the second line, the speaker means that Death arrives, visits, or comes by in order to impose himself on the speaker.

To really consider the difference, compare what it means to "stop for a break" (similar to the idea in Line 1 of the poem) as opposed to "stop by my friend's house" (similar to the idea in Line 2). The word, "stop," is the same, but the meaning is different.

Further, the second appearance of the word "stop" adds increasing weight and seriousness to this opening stanza. By saying "stop for" and "stopped for" so closely together, the speaker establishes a dramatic, emphatic tone; the repetition of that harsh, monosyllabic word "stop" makes us think about the inevitability of death.