Jesus had given the disciples a clear mission: they were to be witnesses for Him to the world. So, while they waited, they prepared for their mission in two ways. First, Luke says they continued in united prayer and supplication. There was no question in any of their minds about what the mission was that Jesus had given them, and they had each accepted that mission. This inspired them to unite in prayer. Luke does not share what they were praying about, but it is most certain they were praying for wisdom, strength, and courage to fulfil the mission together. What an example for us.