Here's why you don't understand the title: it doesn't make sense.
The phrase "Eyes wide OPEN" is an idiom. It means "to begin something difficult, fully understanding the difficulties."
The grammar of "eyes wide open" is that the phrase "wide open" acts as an adjective, describing "eyes." I'm not a grammarian so I can't really analyze "wide open." But I'll note that "wide-open," with a hyphen, is actually a good English word meaning "completely open."
Movie titles are not like section headings in a textbook. They're supposed to catch your attention. This phrase catches our attention because it's the opposite of what we expect. Your first reaction is "Eyes wide SHUT? What on earth can that possibly mean?"
The meaning is whatever you read into it. Nothing in the movie makes it any clearer. Maybe it means that these characters think they are going into a situation "with eyes wide open" but that really their eyes are shut.
Don't expect clear English in movie titles, book titles, or song lyrics.
Here are some other examples of famous titles in English that don't make much sense. They don't make sense when you read the title for the first time, and they may or may not make sense after you know the story.
"A Hard Day's Night"
"For Colored Girls Who Have Considered Suicide / When the Rainbow Is Enuf"
"They Shoot Horses, Don't They?"
"The Unbearable Lightness of Being"
"Yellow Submarine"
"You Only Live Twice"