High Fantasy or Epic Fantasy as it is also called, can be a tricky subgenre to adequately define. Its easy to be too narrow or too broad, leaving out major works or including too many. But I know High Fantasy when I see it.
Here are some of the usual elements of High Fantasy:
• An imaginary world (usually an entirely imaginary world, but can be a parallel world or a world-within-our-world)
• A battle of good versus evil or other epic theme, with terrible consequences if the hero fails
• Multi-volumed works with a complicated plot, a large cast of characters and an expansive timeframe
• Non-human races such as elves and dwarves and/or magical creatures such as dragons and unicorns and/or monsters such as orcs and trolls
• Medieval technology and feudal structures
High Fantasy novels can have all of these elements, or only some.