It's from AI Brooks' Reading Price Charts Bar by Bar.
• H1 –First pullback in an uptrend, above 1st bar that has a high above previous high.
• H2 – Second time a High above previous bar in that same pullback
• L1 - First pullback in a downtrend, below 1st bar that has a low below previous low.
• L2 – Second time a Low below previous bar in that same pullback
Here's a sample of L1/L2 and a typical AI Brooks' scalp. I should have used ES 5 min chart and the sample may not be a best trade for L2 but it should be enough to explain the concept.