Adam Clarke on Hebrews 2:4

God also bearing them witness - He did not leave the confirmation of these great truths to the testimony of men; he bore his own testimony to them by signs, wonders, various miracles, and distributions of the Holy Ghost, Πνευματος 'Αγιου μερισμοις. And all these were proved to come from himself; for no man could do those miracles at his own pleasure, but the power to work them was given according to God's own will; or rather, God himself wrought them, in order to accredit the ministry of his servants.

For the meaning of signs, wonders, etc., See the note on Deu 4:34.