2 Corinthians 2:12 (NKJV)
Doors can be found in many different places, there are many different types of doors, and, usually, all doors can be opened and closed. If a door is closed, and someone wants to go through the door, then someone has to open the door. In some places, certain people are given the job of opening a door, and there are some doors that only certain people have access to. Doors lead to rooms, hallways, and other areas, but if a door is not open, no one is able to go through it to what is beyond the door.
In 2 Corinthians 2:12, Paul talked about going to the city of Troas to preach the Gospel, and how God had opened a door for him, God opened up an opportunity for Paul, which most likely had to do with Paul preaching the Gospel. Paul could have said that he opened the door himself or that someone else opened a door for him, but Paul said it was God that opened the door. Paul went to Troas with the goal of preaching the Gospel, but the opportunity Paul had there in Troas was given to him by God.
Christians are sometimes like Paul where they have a goal in mind, but they need an opportunity, they need a door to open for them. Too often, people try to create a door, or open a door themselves, and they do not have the success that they want or should have. The best thing is that we know what we should do, and we are prepared to do it, but we allow God to open the door of opportunity we need, to do what we plan to do. We need to trust God, not only to follow His direction in our life, but to allow Him to open the door we need, to be able to do what He has directed us to do.