In this block you load a custom indicator and you choose one buffer. I don't care what the indicator is and what is the number of the buffer, this block does the same. It expects the values of the buffer to be EMPTY_VALUE or 0 most of the time, and only from time to time to be something different, lets's say the price value.
The idea is that every buffer has as many values as many candles there are on the chart. If each value is a positive number that can be seen on the chart, then you see something like MA line. But some indicators don't want to draw continuous line, so they use that EMPTY_VALUE value, which is a MQL constant that represents the biggest possible integer value.
"Indicator appear" checks the indicator every time it runs and remembers its last checked values. If the last checked value was EMPTY_VALUE and the current value is say 1.2345, it passes.
I also set it to work with value 0, because some indicators use 0 instead of EMPTY_VALUE.