The novel focuses on the Igbo people in the years leading up to the arrival of the first Christian missionaries—it follows Okonkwo, a respected member of the tribe, as he builds up his reputation; knocks it down again with some accidental murders; loves, raises, and kills an adoptive son; and is exiled for killing a different boy. Basically, we get to see the ins and outs of pre-colonized Africa and get a hint of what's to come when the missionaries show up.
So here's Achebe's quandary: how does he remain a hardcore critic of a colonial regime that imposes its language on a native population when he himself writes (and writes so well) in English—the tongue of the colonizer?
These days, most scholars read Achebe's quandary as what it is: a problem that greets most postcolonial writers who have no real choice but to write in the dominant (or "'hegemonic"') language since that's what they were taught.
The real question becomes: does the writer (in this case, Achebe) do anything to question, "'problematize,"' or revolutionize that dominant language so that we—as readers—are aware of the problems that come with writing and speaking the "'master's language"'? And, for the most part, postcolonial scholars do get that Achebe tries something different with language in his novel.
The way he incorporates the language of the Igbo people into the story itself is often seen as his attempt to put a "'native"' language other than English front and center. That way we get to see what we are missing as English readers.