The adjective beautiful describes a thing that is pleasing to the senses. A field of wildflowers, a colorful sunset, and an abstract sculpture could all be considered beautiful.
The word beautiful is closely related to the words beauty and beautify, and you'll notice they all share the same beginning, beau-. Almost anything can be described as beautiful because, as the saying goes, "Beauty is in the eye of the beholder." So while you may think the brown spotted grass frog is unattractive, a scientist who spends all day studying them may think it's beautiful.