Vocal clarity mostly lives in the 500Hz–2.5kHz range covered by this tool's two middle bands, which is why they're the first place to look when a voice sounds buried or harsh.
If vocals sound muffled, try a small boost around 2.5kHz. If they sound harsh or sibilant instead, a gentle cut in the same region often fixes it — the direction depends entirely on the specific recording, so trust your ears over any fixed rule.
Lower male voices often benefit from a touch more warmth around 500Hz, while higher female or child voices more often need a cut rather than boost in the high-mid range to avoid sounding shrill — listen for which direction actually helps before committing.