Numbers 11:25 And the Lord came down in a cloud, and spake unto him, and took of the spirit that was upon him, and gave it unto the seventy elders: and it came to pass, that, when the spirit rested upon them, they prophesied, and did not cease.
The Lord took of the spirit that was on Moses and gave it to the 70 elders. Their response to the moving of the Spirit upon them was that they prophesied. This was apparently the visible evidence of an invisible occurrence. The burden of leadership was heavy upon Moses, so some of the responsibility was shifted to the 70 elders. The Lord qualified them for the job by giving the Spirit. How would the people know? They prophesied.
To prophesy is to speak the words of God to people. God uses the prophet as a mouthpiece to say what He wants said. We do not know what they said or did in this instance. We do not know what Samuel and the school of the prophets said when they prophesied. We do not know what Saul said when he prophesied among the prophets. We do, however, know what the 120 said on the Day of Pentecost when the Spirit was poured out in all flesh. Acts 2:11: “we hear them telling in our own tongues the mighty works of God.”
This is most likely what prophesying is: telling about God, or proclaiming the gospel. When the Spirit moves us we are moved to talk about God and tell others what He has done. We can expect that in certain situations the Holy Spirit will give us words to say. This kind of response is what Spirit-filled believers will do. They will do whatever is needed to support the proclamation of the gospel through the local church or through mission work.