To Polish, To Shine, To Brush Your Teeth
transitive verb, godan verb
Because the hiragana ends with an う sound, you know this word is a verb. The kanji itself means polish and the verb vocab version is to polish or, strangely specifically (and we'll learn an even more specific word for this later) to brush your teeth.
みがく
The reading is the same as the one you learned with the kanji.
何で切れた電球なんか磨いてるの？
Why are you polishing a light bulb that’s already burnt out?