Equalisation

An EQ splits the sound into frequency ranges and lets you change the level of each one. On a voice the standard moves are a high-pass filter to clear rumble below the speaking range, a cut around the low mids where boxy room sound gathers, and a gentle lift higher up for clarity.

Cutting is safer than boosting, because a boost raises whatever noise already sits in that band along with the thing you wanted. Work in small amounts, listen in the context of the full mix rather than solo, and go back to the recording chain when a problem needs more than a few decibels of correction. No amount of EQ fixes a bad room.