When you're looking or feeling wistful, you're a little sad because you really want something that you don't have yet, or because you really miss something that you don't have anymore. You long for something. You yearn for something. And you don't have it, so you're wistful.
You can feel wistful and hopeful at the same time, if you think you might get what you want, or you know you will get it. If recess is an hour away, and you sit at your desk, gazing wistfully out the window at the monkey bars, then you know you'll definitely get to play soon; in the meantime, you're full of sad longing. You're wistful.
But you can also feel wistful and hopeless, if you think you won't get what you want. If your team just lost an important game, and you stand by your coach, gazing wistfully over at the winning team as they celebrate their victory, then you wish you would have won—but you know you didn't. You might sigh wistfully.
Either way, hopeful or hopeless, wistfulness is a sad, serious, thoughtful kind of feeling or mood.
When you look at the word "wistful," you might think, "Okay, something beautiful is full of beauty, and someone careful uses a lot of care, so someone wistful must have a lot of wists... and, uh, what are wists?" Well, there's not a great answer! Sorry! Long ago, "wist" meant "attention," and so if you were wistful, you were paying a lot of attention to something, watching for it and, perhaps, hoping to see it. But over time, probably because "wist" sounds like "wish," the meaning of "wistful" changed from "attentive as you long for something" to just "longing for something."