Conflict is the struggle that a character in a plot experiences. It is what propels the story forward. There are two kinds of conflicts that a character may face in a plot.
External conflict is the one that a character faces from an outside force. This outside force may be another character, a group, a turn of events or nature.
Internal conflict is the one that the character faces from within himself. His mind or heart may create a situation of a struggle for the character in the plot.