This line is spoken by the character Leslie in the movie based on the novel Bridge to Terabithia, but it's not actually in the book. When a book gets adapted into a movie, writers often make a lot of changes, deletions, and additions--and this is something that was added in, probably to make Leslie's character even more free-spirited and inspirational to Jess, whose imagination is set free thanks to her.
In particular, by asking Jess to close his eyes but still have a "mind wide open," Leslie (the film version of the character) is instructing him to stop observing what's actually around him and instead allow his imagination to take over. Keeping an open mind also means not judging anyone or anything, so Leslie also wants Jess to not worry or make judgments--just play, and imagine, and have fun.
We can tell that Leslie's attempts to open Jess's mind and set his imagination free actually do work. How?
Well, we see Jess having fun with imaginative play with Leslie in "Terabithia" when they're playing together. But when Leslie is too busy to play, toward the middle of the story, Jess isn't yet ready to play on his own:
"Jess tried going to Terabithia alone, but it was no good. It needed Leslie to make the magic. He was afraid he would destroy everything by trying to force the magic on his own, when it was plain that the magic was reluctant to come for him."
But then, through Leslie's influence and through his own journey of self-discovery, by the end of the book, Jess is able to play imaginatively without Leslie:
And when he finished, he put flowers in [his sister's] hair and led her across the bridge - the great bridge into Terabithia - which might look to someone with no magic in him like a few planks across a nearly dry gully.
"Shhh," he said. "Look."
"Can't you see 'em?" he whispered. "All the Terabithians standing on tiptoe to see you."
"Shhh, yes. There's a rumor going around that the beautiful girl arriving today might be the queen they've been waiting for."