Socks that are at "ankle length".
Despite the name, the tag refers to socks that are sightly above the ankles, not the extremely short ones that stop exactly at or below the ankles. They end before the shins, if they go over the shins, they become kneehighs.
Not to be confused with booties, a sock-like garment that is also ankle length.
See post #86570 for reference, read the translation notes.