The gospel is the good news that God acts through Jesus to rescue and renew us - and all of creation - from sin and its effects.
The world, for all its beauty and life still leaves us with strong evidence that something is wrong. We are surrounded by death, disease, sadness, fear, violence, etc. This is not the way it's supposed to be and deep down we all know it.
According to the Bible, the reason for all the brokenness is ultimately traced back to sin. People have rebelled against God’s rule and have instead set themselves up to be their own “gods” who direct their own lives the way they want according to their own standards. This rebellion is sin. So sin is the ultimate problem and it's a probelm we cannot work our way out of. In fact, the Bible says it this way, "The wages of sin is death." (Romans 6.23)
Because people are unable to work themselves back into an acceptable position before God, God graciously sent his Son, Jesus into the world to rescue people from their sin by paying the penalty for our sin. This payment came in the form of Jesus lovingly laying down his own life by dying for our sinful rebellion. He became our payment for sin.
But after Jesus was crucified, he was raised back to life. The power of God was demonstrated in the resurrection and Jesus now stands over death in triumph. Through the death and resurrection of Jesus Christ the power of sin has been defeated.
Because of Jesus' work, anyone who repents and believes on him for salvation (rather than their own efforts) receives forgiveness of sin, restored relationship with God, freedom to worship God, power to overcome sin, and the promise eternal life with God. This is why the gospel is often called good news. It really is good news for those who trust in Jesus.