Indicates a very small amount or degree; 'mere', 'only', 'just'. Usually written in kana in modern usage.
ほんの少し手伝ってくれませんか？
Could you help me just a little?
これはほんの気持ちです。受け取ってください。
This is just a small token (of my appreciation). Please accept it.
彼の言ったことはほんの冗談だったから気にしないで。
What he said was only a joke, so don't worry about it.