What is Monergism?
Monergism is a word that comes straight from Greek, and means "one working," as opposed to more than one contributing to doing something. The beginning of our salvation, our coming alive spiritually, usually called regeneration, is monergistic. God revives us by His Spirit. We do not contribute to this in any way. When Jesus called Lazarus from the tomb, Lazarus wasn't pushing on the door trying to get out. God alone worked life into him by that call. Because of this, God gets all the glory in our salvation.
Justification can also be described as monergistic. God provides the righteousness and sacrifice of Jesus as the ground for our forgiving our sins, grants us faith to trust Jesus, and then declares us righteous in His sight.
After this beginning, there is synergistic activity - both God and us working. The work we must do is to believe in Jesus, accept the truth about sinful ourselves as shown in the Bible, and strive to live a godly life from then on.
Many evangelicals want to bring our work of believing at the beginning of the process. "Believe and you will be born again," they say. They usually do this out of a good motive to emphasize the importance of our acting, our believing and committing our life to Christ. But it's still putting faith in the wrong place. You can't believe until you're alive, and we are spiritually dead before God makes us live.
Again, we need to affirm this so God gets all the glory in saving us. If it's my decision that makes the difference, then I can claim credit for being saved. Even though you're trusting Jesus to do all the saving, you can compare yourself favorably with unbelievers. And your decision is the difference, not God's grace. Monergism keeps us away from this. Any gift we have, including our faith in Jesus, is because of God graciously giving it to us.