How is the run time calculated in this insertion sort algorithm? I am looking at this algorithm in CLRS page 26:

I am a novice when it comes to these things so please make your explanations as simple as possible.
For the cost column, does c1, c2, c4, etc denote a value the same way n does, or are they just labels? My understanding is an operation takes one time unit (such as j=2 is an operation that writes something once, therefore its one time unit) so does c1 mean one time unit?
Secondly, why does key=A[j] happen n-1 times?
Thirdly, line 3 is a comment therefore does not have an execution, so why does it happen n-1 times?
Let me answer step by step:

$c_1, c_2, c_4$ etc. are costs of the operation in the corresponding line. For example $c_1$ is the cost of for: one pass of this line costs $c_1$.
$key=A[j]$ happens $n-1$ times, because we start from $2$ and $n=A.\text{length}$.
As it is commented, its cost is $0$.