A high-efficiency transmission built to survive a worse channel than the ordinary single-user format.
Reading a capture
Put this beside he-su on the comparison page and the difference is one longer bar. That bar is what extra range costs: not power, not a different modulation, just the same signal field sent a second time.
Subfields
HE-SIG-A — The same fifty-two bits the single-user format carries, sent over four symbols instead of two.