The speaker begins by using the first-person plural pronoun, saying, "We shall not always plant while others reap." Considering that the poem is about how Black people will someday overcome racism and oppression, the speaker's use of "we" is notable because it establishes that the speaker is Black.
Of course, the speaker never explicitly states that this poem is about Black oppression, but the opening two lines make this element clear enough since the speaker alludes to the institution of slavery by referencing the act of planting. The main kind of forced labor during slavery was agricultural, meaning that most enslaved people were forced to work in fields—fields that belonged to white enslavers who then "reap[ed]" the benefits of this labor.
This first line also alludes to a proverb found in the Bible, which says that "whatsoever a man soweth, that shall he also reap." This phrase has worked its way into popular usage, though the wording is usually changed to something like, "You reap what you sow." It essentially emphasizes the idea that hard work leads to positive outcomes, since the act of sowing—which means planting—is what creates harvestable crops.
However, the poem turns this phrase on its head by spotlighting the ways in which Black Americans have been unable actually "reap" what they sow. Indeed, the speaker's point is that Black Americans have historically been cut off from enjoying the fruits of their labor. To that end, the oppression of Black people has created a situation in which powerful white people are the ones to benefit from the hard work of Black people.
Despite this, the speaker is confident that this will not always be the case. In fact, the poem begins with the speaker's assertion that Black people will "not always" be mistreated in this way. In keeping with this, the speaker is confident that Black people will one day be able to enjoy "the golden increment of bursting fruit," which is the speaker's way of saying that Black people will someday actually benefit from the precious growth of the plants they themselves have worked so hard to tend.
It's worth noting that the consonance in these first two lines is very euphonic, since there are so many pleasing, rounded or humming sounds, like /l/, /th/, /z/, /n/, /m/, and /r/:
We shall not always plant while others reap
The golden increment of bursting fruit
This euphony gives the opening lines a pleasant, lush sound that ultimately matches the speaker's highly poetic tone. In keeping with this poetic tone, these lines also establish the sonnet's adherence to iambic pentameter, meaning that each line contains five iambs (metrical feet made up of an unstressed syllable followed by a stressed syllable, da-DUM). The lines scan like this:
We shall | not al- | ways plant | while oth- | ers reap
The gold- | en in- | crement | of burst- | ing fruit
These lines are good examples of iambic pentameter, but it's worth mentioning that some readers might hear a stress on the word "not" in line 1, turning the line's second foot ("not al-") into a spondee, meaning that both syllables are stressed. If this is the case, then the speaker ultimately ends up emphasizing the idea that Black people won't live in oppression forever.