vim-airline requires you to use a patched font that replaces certain little-used unicode characters with shapes that airline uses to draw its UI. Specifically, those < and > looking dividers in the airline statusline. Although it's totally possible to use airline without this and it will look only slightly less fancy.
They're not little-used unicode characters. They're code points in the Private Use Area, which is an area of Unicode that's explicitly set aside and will never be assigned to "real" characters, specifically so they can be used for custom things. vim-airline's use of the PUA for its custom glyphs is quite appropriate, it's just annoying that it requires a patched font.
Incidentally, the Apple glyph (, or ⌥⇧K on macOS), is actually in the PUA as well (in fact, it's the very last PUA codepoint, U+F8FF). Which is why it may not render correctly in fonts that don't ship on macOS. Anyone reading this on Windows, Linux, or Android probably won't actually see the apple character.
