"And being assembled together with them, He (Jesus) commanded them not to depart from Jerusalem, but to wait for the Promise of the Father, 'which,' He said, 'you have heard from Me; for John truly baptized with water, but you shall be baptized with the Holy Spirit not many days from now." Acts 1:4-5
Before Jesus death and resurrection, His disciples were not commited or faithful when they were placed under pressure. When being told to pray in the Garden, the pressures of sleep got the best of them and instead of praying they shut their eyes and drifted into dreams. Even after Jesus had resurrected, on the Mountain of Galilee, He had given the disciples what we call, 'the Great Commision' of making disciples all around the world and baptising them. Instead, Peter influences some of the disciples to go fishing.
Jesus in this Scripture gives His disciples another command after failing to keep many of His commandments.
This time though, Peter and the disciples decided to carry out the order that Jesus had given. The order was to wait in Jerusalem for the baptism of the Holy Spirit. And in carrying out this order, the world was turned completely upside down for the good. In their simple act of obidience, the Spirit came and made a huge impact to Jesus' disciples and to the Church.
Instead of struggling in the flesh, the usually disobient disciples were given power in place of their weaknesses to do a work for God.
The reason why Peter was waiting in Jerusalem may have been a mystery to him, all he knew was something about a 'Spirit'. But it was all clear to him when the Spirit came, it shows because he spoke about it in Acts 2. He was even able to address the questions that had grieved many of the believers at that time; why Jesus died, when Jesus resurrected, why Jesus did not establish His kingdom at that time and why He ascended instead of stayed.
When the Spirit came and used Peter to address these burdensome questions, the scriptures say many were 'cut to the heart' and asked, 'what should we do?'. Peter responded with what was never really clear to him before, but clearly understood now, and could explain perfectly. He responded by saying, "Repent and let everyone of you be baptized in the name of Jesus Christ for the remission of sins; and you shall receive the gift of the Holy Spirit. For the promise is to you and to your children, and to all who are afar off, as many as the Lord our God will call."(Acts 2:38-39)
Something we can learn is that like Peter we need to finally decide to obey by faith, especially when we don't understand. In time the Lord will accomplish what He set out to do. In turn, we'll bless others as we share with full understanding what God has done. People will be cut to the heart, people will be blown away and people will be saved.