"Gospel" is a word used in the Bible that means "good news." Specifically, it refers to the good news that Jesus Christ, the Son of God, has come to earth and done everything necessary on our behalf to restore us back to God. He did this by living a perfectly obedient life that we were unable to live because of our sin, dying a sacrificial death in our place to pay the debt for our sin, and rising again three days later to defeat sin and death, proving that He was indeed the Son of God. The video below helps explain this in more detail.