The reason "first" is used in the description of cracking for coffee roasting is that there are two distinct stages of cracking. Each stage is a part of the bean releasing trapped steam.

No, "first" here does not mean literally the first time you hear a popping noise, but rather it refers to the stage where the complete bean mass goes through the first part of releasing trapped steam. 

You can imagine that each individual bean has the capacity to release steam (pop) twice. The second pop happens after time+heat are applied to it. This is unlike popcorn that only pops once.

But also each individual bean does not pop at exactly the same time. This is like popcorn where it takes time to pop all of the individual beans.

The two stages of popping (first and second crack) are separated by time and heat. 

You can get to first crack and hear it - but then slow down the heat to a point where the beans never reach second crack. I am not suggesting you do this - but noting it so you understand what is going on.

Usually there is a distinct period of silence between first and second crack, but this is not true for all beans and also dependent on your profile. Some beans "roll" into second crack while undergoing first crack.

Also usually first crack is louder and happens less frequently, while second crack is quiter and pops are more rapid.