It's one level of abstraction above the transistor level; in other words gates and sets of gates. Designing at the raw transistor level is tedious, time-consuming, and requires specialized knowledge of low-level electrical and physics characteristics. When you design with standard cells, you only have to think about the logic functionality.
The parent's question is about whether Apple really did do some of the designing at the transistor level by designing their own cells, rather than using a standard library. This could give them an edge that ordinary users of the standard library wouldn't have.
"A standard cell library is a collection of low-level electronic logic functions such as AND, OR, INVERT, flip-flops, latches, and buffers. "
I think SRAMs are also built of standard cells & there can be a lot of flexibility to improve on the fab's library, particularly with multiple ports for caches.
