The term "cornerstone" refers to an important stone that stabilizes a building, usually by holding together two interlocking rows of stones.
So what do those numerous passages in the Bible referring to Jesus as the Cornerstone mean? Just that. He is our stabilizer. Our main support. Without Him, everything would fall apart.
The gospel message is really not that difficult, if we can grasp the cornerstone of our faith. Ann Spangler writes that often "we cannot quite believe that the good news is as good as it is. So we try to add to it...let's get it straight today. The cornerstone of our faith is this: if we confess with our mouths, 'Jesus is Lord,' and believe in our hearts that God raised Him from the dead, we will be saved."