These are incredibly important words!
The 4 different gospels each have accounts of Jesus' ministry; the accounts all agree, but sometimes it is hard to find out what happened first.
In the book of Mark, the first words that it records Jesus as speaking in His public ministry are these:
"The time is fulfilled, and the kingdom of God is at hand; repent and believe in the gospel."
These are wonderful words! Jesus is saying that the time is fulfilled for Him to be here on earth and the kingdom of God is at hand. Jesus Himself, the Son of God, was now, here on earth!
What were people to do? They were to repent -- to turn from their sins -- and to believe the good news.
Those simple instructions are still the words today to the whole earth. What are we to do? We are to repent -- to ask forgiveness for our sins and to turn to God -- and to believe the gospel, the good news that Jesus Christ is Savior and Lord and died for our sins.