For three and a half years, Jesus had taught His disciples the principles of the Kingdom of God and their roles in establishing it on earth.
Yet, before they could fully launch into this Kingdom assignment, Jesus emphasized the need for empowerment. He told them that they would receive enabling power through the help of the Holy Spirit, the promised Comforter:
Acts 1:8
But you will receive power when the Holy Spirit comes on you; and you will be my witnesses in Jerusalem … and to the ends of the earth”
The word power here simply means all the help and aid the Holy Spirit provides, including the ability to speak in new tongues, preach the gospel effectively, endure trials and produce signs and wonders.