‘And coming to us, and taking Paul's girdle, he bound his own feet and hands, and said, “Thus says the Holy Spirit, So will the Jews at Jerusalem bind the man who owns this girdle, and will deliver him into the hands of the Gentiles.” '
Agabus deliberately sought them out and then took Paul's belt and used it to bind his own hands and feet. And then he declared that the Holy Spirit had shown him that the owner of that belt would himself be bound in the same way by the Jews in Jerusalem, and would then be handed over to the Gentiles. This last would be seen as the worst possible fate for a Jew. He would be unable to maintain his religious cleanliness and would be cut off from Israel.
We note that this is the third time that Luke has mentioned these warnings, indicating completeness of warning (Acts 20:23; Acts 21:4). He was in fact warned any number of times (Acts 20:23). This acted out prophecy of Agabus relates him to the Old Testament prophets who regularly acted out their prophecies (1 Kings 11:29-31; Isaiah 20:2-4; Jeremiah 13:1-7; Ezekiel 4:1-17).