Five thousand men, women and children were hungry.  They had been listening to Jesus all day and He told His trainee apostles to feed the people.  They failed the test, even with Jesus' instruction and authority (Luke 9:13).  So, then Jesus stepped in, making sure that the disciples were not just passive observers.  This miracle was more to teach them than to satisfy a hungry crowd.  Jesus ordered the disciples to sit the people down in manageable groups, and they did what they were told, accepting Jesus’ authority over them – the first essential step in faith (Luke 7:8-9).  

Peter had just declared that Jesus was the promised Messiah (Luke 9:20) – the Son of God, specially anointed to represent heaven on earth.  But this new realisation of who Jesus was became the beginning of the road to the cross, because even religious people would prefer God to be remote, and not close up and personal.  Religious leaders liked to think of themselves as 'brokers' between God and the people, and they enjoyed all the perks of the 'middle-man'.  But now it was clear, at least to the disciples, that Jesus was not just another religious 'broker': He carried the full authority of

The early days of being with Jesus must have been very exciting for the disciples.  But now the reality of His mission (and theirs) was being spelled out.  In order to fulfil God's plan of defeating the bad works of Satan (1 John 3:8), and rescuing people who have been held in his grip (Hebrews 2:14-15), Jesus would have to sacrifice His own life (Matthew 16:21).  

Some people seem to have an urge to respond to something they do not understand, by speaking, however inappropriately.  That was Peter.  The glory covering Jesus and the appearance of Moses and Elijah was way outside his experience, yet he proposed a response.  Instead of closing his mouth and bowing in awe, he stood up and spoke foolishly.