Mar 1:15 The time is fulfilled, and the kingdom of God is at hand: repent you, and believe the gospel.
The measure is full and the kingdom of God is brought near; you can change your mind and believe in the reward for bringing good news.
These are Christ's first words in Mark. They are more forceful and direct than Christ's first words in Matthew. Here he provides a simple summary of his mission.
The first idea, that is that the time has come and the measure is full (both ideas are the same) suggests that human history had to get to a certain point before it was ready for Christ's message. That time was predicted to come and it did. Of course, the big issue is what that change entails.
Next it moves to the very difficult idea of "the kingdom of God." (Interestingly, Mark starts with this formulation referring to "God" instead of the more common phrase of "the kingdom of heaven." I don't know that the two phrases means exactly the same thing.) In either case, this is a central idea because Christ spends most of his time in Matthew explaining this idea and a difficult one. While this idea may encompass aspects of the afterlife, it clearly is more than that: a state of being that is aligned with God, an idea and culture spreading through human civilzation. Finally, it ends with a statement that I think has been mistranslated.
"Good news" has come to mean the Christ story itself, i.e. the Gospel, but Christ doesn't seem to use it that way at all. He used it more simply. The phrase doesn't mean the "good news itself." Instead it means "the reward for bringing good news." For Christ, changing our mind means believing that trusting in the future and specifically in a future that is increasingly close to God, will have its own rewards. This is at one level, a statement about optimism generally. Christ is saying that the world starts to get better when we believe that we will be rewarded for bringing good news instead of punished for it. This goes beyond simply optimism because the good news he brings is specific: that God is changing the world through each of us.
ὁ καιρὸς "Time" is from kairos, which means "due measure", "proportion", "fitness", "exact time", "season", "opportunity", "time", "critical times", "advantage," and "profit." It is the concept of time as a moment as opposed to a measurement. The ideas of good times or bad times as a part from seconds, minutes, and hours.
τῷ εὐαγγελίῳ. "Good news" is from euangelion(euaggelion) a "reward of good tidings," a "thank offering for good tidings, " "good news," and "good tidings." Originally, this terms described a reward, like a tip, given to a messenger who brought good news.