On Google Fonts they allow you to filter between Serif & Display fonts, I've only ever known about Serif vs SansSerif.
Are display fonts just fonts inteneded for bigger font sizes and can either be serif or sans-serif?
Display fonts are intended to be used for larger sizes. The spacing, thickness, serifs, etc are optimized to improve legibility in large displays and make the subtle differences in shapes easier to see. Both serif and sans-serif fonts may have display variants.
Display fonts are commonly used for signage, posters, and other large print media. They are also sometimes used for headings in text.