The NABRE, in its footnote commentary on Mark 2:1-2, says, in part:
preached the word , i.e., the gospel concerning the nearness of the kingdom and the necessity of repentance and faith (Mk 1:14). (
source)
Which likely boots back to the Old Testament and those passages which speak of the Messianic prophecies. Christ was a Jew, and at that time and place it was very common to speak in what appear to be riddles; much of what the Gospel writers report of Christ saying to others was to draw them out as to what they thought. He wasn’t like “Dude! It’s clear!” , but rather challenged them with the Scriptures they knew, and with His presence and actions.