To achieve an intended tone, writers have several tools at their disposal, like choosing whether to use the active or passive voice. Simply put, the writer decides whether a person (or a thing or idea) did something, or whether something was done to a person, thing, or concept.
Did I do (something), or was something done (by me)?
For example, "I convinced him" is active voice, while "he was convinced by me" describes potentially the same situation in the passive voice. "She drove the car" is active voice; "the car was driven by her" is passive voice.
Note too that active and passive voices differ in terms of what information can be left out; "I conquered" (active voice) doesn't need to specify the defeated party or the matter at stake, while "our land was conquered" or "we were conquered" (both passive voice, with differing meaning) leave out who or what did the conquering.
Why does this matter? Emphasizing, de-emphasizing, or even leaving out the active agent, or the agent acted upon, has a major impact in how a piece of writing comes across to the reader. For example, in some forms of organizational writing, it is still possible to read pages of text written in the passive voice, in which many things are done or steps are taken without a clear sense of who, or what group, is acting.
This decision is important, but for academic writing there are usually clear audience expectations that almost require the use of active or passive voice. For more than a generation now, most scholarly writing has favoured the active voice, the clear identification of subjects who do particular things rather than a sequence of processes happening to passive objects. However, the opposite is true in descriptions of methodologies, where many writers continue to employ an impartial set of instructions that could be followed by any researcher, rather than emphasizing themselves by composing a narrative of their actions.