Matthew Henry's Concise Commentary 3:22-36 John was fully satisfied with the place and work assigned him; but Jesus came on a more important work. He also knew that Jesus would increase in honor and influence, for of his government and peace there would be no end, while he himself would be less followed. John knew that Jesus came from heaven as the Son of God, while he was a sinful, mortal man, who could only speak about the more plain subjects of religion. The words of Jesus were the words of God; he had the Spirit, not by measure, as the prophets, but in all fulness. Everlasting life could only be had by faith in Him, and might be thus obtained; whereas all those, who believe not in the Son of God, cannot partake of salvation, but the wrath of God for ever rests upon them.
John was saying that his time had come to die, he full filled the assignment that was given to him, he was a prophet, the forerunner, for Jesus of Nazareth; and when his job was done; he knew he was going to die(decrease) an Jesus was about to increase ( start his ministry) that's what the meaning of John 2:30. means.
Its a fancy english way of saying what John probably said more simply - that he needed to become less known to the people and Jesus more well known. John knew he was merely the prologue to the greater story. He knew his purpose was to prepare the way for Jesus not to have his own following and he knew when it was time to step aside and hand things over to Christ.
you will be able to understand what John the baptist means if you continue to read it till verse 31
let us read,
31 He that cometh from above is above all: he that is of the earth is earthly, and speaketh of the earth: he that cometh from heaven is above all.
Jesus is above all because he cometh from heaven, though John the baptist is consider as a great propeht/person but under the power of the Lord Jesus he must decrease so that the faith of his disciples (john's disciples) will be switched over to Jesus since Jesus is greater than Him.
John the baptist is showing humbleness in this statement and he wants to tell to people that people must believe in christ than him.
He must decrease for the poeple to see that Jesus is greater than him.