It's one of those basic ingredients we teach and learn in the classroom, and yet it's surprising how many beginning writers forget to mention this. They may have an overarching problem their main character has, but they get lost in mundane details when they write the story and forget the struggle the character needs to face in the moment.
Writers often love their characters, sometimes to the point of distraction. They don't want bad things to happen to them, but if the character isn't embroiled in some dilemma, what reason does a reader have to go on reading?
Does this mean the character must be in constant peril? No. Conflict can be subtle, it doesn't have to mean the antagonist is pointing the gun at him for the whole story. This can be just as dangerous to a story than having no conflict. If the character goes from one high conflict moment to the next, it can exhaust the reader because there is no down time or reflection allowed before the next intensity happens. It's like a roller coaster. The ups and downs of a story give it balance and pace a reader wants.
Remember there are many types of conflict. In some scenes it will be overt, a life or death situation. In others it will be understated, an internal question the character must decide.
Conflict involves characters not getting what they want or desire and facing some impediment to achieving their objective.