Hi,
so basically you are looking at one of the applications of iterators (previously known as adverbs). This particular form can be compared to a "Do" loop in other programming languages. Let's have a look at the code, for simplicity let's define the function f as following
f:{x,sum -2#x}
Then your code becomes
10 f/ 1 1
On a side note this could also be written as f/[10;1 1] which might make it easier to understand. In this case / (over) acts as an accumulator, executing your function f 10 times using 1 1 as initial parameter. the result of the first iteration then becomes the input of the second iteration and so on.
what does f do?
q is executed left of right, means from right to left. -2#x takes the last two elements of x (1 1), sum sums them and the result gets concatenated to the initial input x, thus after your first iteration you get 1 1 2. This will now become the input to your second iteration. A helpful way to understand over better is to use scan \ instead of over. Scan does basically the same as over except it outputs the intermediate steps. The result of over is the last output of scan.
q)10 {x,sum -2#x}\1 1
1 1
1 1 2
1 1 2 3