L-SIG(Signal), starts with 'L-', which stands for Legacy, which is similar to the same fields in 802.11a/b/g and 802.11n.
Fields are duplicated over each 20 MHz sub-band with appropriate phase rotation. Subcarriers are rotated by 90 or 180 degrees in certain sub-bands to reduce PAPR.
L-SIG field includes information regarding the length of the rest of the packet. This means that all devices including the legacy devices will know that a packet of a given length is being transmitted.