A conflict is the problem of the story, novel, or play. It causes the action. There is no story without a conflict. The conflict can be external or internal. An external conflict could be something like war and survival, racism, another person, nature such as a hurricane or flood, or society: something outside of yourself. An internal conflict is within yourself. It could be whether to cheat on a test; tell a secret to help a friend; kill a person; determine what is right or wrong. It goes on inside the head of the individual.
The conflict is the element that adds excitement and suspense to the story. All details beyond the conflict lead to its resolution. They lead to the climax, which is the part of the story that solves the conflict.
For example: In a murder mystery, the conflict is finding the person who is the murderer. The climax would be when that murderer is caught. The climax is where the conflict is resolved.
There may be more than one conflict going on in a novel or play. In a short story, it is usually one conflict because it has to be solved in a short space. However, in a novel, there are more characters, a longer period of time, and better characterization. A character could have an internal and external conflict. Or two characters could have conflicts. However, one conflict will take precedence and will direct the course of the story.