L-LTF, starts with 'L-', which stands for Legacy, is L-LTF (LongTraining Field), 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.
The L-LTF, along with L-STF(Long Training Field), contain information that allows the device to detect the signal, perform frequency offset estimation, timing synchronization, etc. The 'L-' stands for 'legacy' and the details of the sequences used in these fields for the 20 MHz signals are the same as the legacy 11a and 11n preamble fields which allows for all 802.11 devices to synchronize to the signal.