With the MACD indicator, the zero level is where the fast moving line (12) crosses the slowing moving line (26). In the chart below, I have added a 12 EMA and 26 EMA to the price chart so you can understand the calculation. As I have mentioned previously, I do not use MA crossover signals as operative buy/sell signals, but they are useful at helping to define the trend. So, the zero line (highlighted in lower scale with red horizontal line)
signifies the level where these two EMA lines meet and cross one another. Oftentimes, good signals take place near this level. The chart below is the same daily chart of Herbalife (HLF) only with the 18 and 40 simple MA lines back with the price chart.
Notice that in February, MACD crossed down through the zero line at almost the exact same time that price broke its trendline. This is the start of the change of trend sequence and the rally back in March sets up the classic sell signal following the 18 and 40 MA line crossover to the downside. Zero line reversal signals take place when the MACD line itself rallies back toward that key level and turns down. The is a great example because it shows the power of having confluence amongst the price structure (1-2-3 change in trend), the MA pattern (classic sell signal) and the failure of the MACD line near its zero level. When you can build a case with independent signals, your odds will improve.
The daily chart of Delek US Holdings Inc. (DK) above shows the zero line reversal in a different form. This rally toward zero signifies the counter trend rally on this time frame vs the downtrend on the higher time frame (weekly). Notice the 1-2-3 sequence that takes place as well as the buy signal that fails using the 18 and 40 MA lines. MACD crossing down through its Signal line is the operative sell signal here and gives the investor a great timing tool to use in conjunction with the other indicators. I cannot stress enough the skill of recognizing the price structure together with the MA setups to give you the understanding of where a stock is situated in its price trend. Then, adding the MACD will improve the timing and confidence even more.
The chart of Sysco Corp. (SYY) is another example of a zero line reversal which helps with the timing of a lower time frame entry. The red arrow points out the actual MACD cross of the Signal line following the zero line reversal.