## Let's say that:
## the number of lines in your file is 200 (range=200);
## you want 30% of the lines in your file: percent=30.
## So we'll have that lim= 200 * 30 / 100 = 60.

for ((i=0;i<lim;i++)); do
## We start our first step of our loop with i=0. We check that 0<200. It is, so this time
## we run the loop.
## At the end of the loop i value will be increased by one: this is the meaning of i++.
## Let's say that now i=15.

num=$(( $RANDOM % $range + 1 ));
## $RANDOM looks like a parameter, but you better think of it as a special function. 
## Every time you call it, you get a pseudo random number between 0 and 32767.
## Let's say this time we get 8512. We calculate the remainder of the division
## 8512/200. So: 8512/200=200*42+112. 112 is the remainder. We add 1, and we get
## num=113. Note: num we'll always be between 1 and 200.

arr[$i]=$num;
## We save this value as arr[15], the 16th element of our array.  At the end our array will have
## lim elements, so 60 elements in this example, indexed from arr[0] to arr[59].

for ((j=0;j<i;j++)); do
## This is our control loop.
## Now we want to check if the 16th element we've just found is a repetition. We 
## already know that our first 15 elements are all different, because we've already run
## this same test for each of the past 15 elements.

(( ${arr[$j]} == $num )) && {
## So we compare arr[15] with arr[0], then with arr[1], then..., then with arr[14].
## If at any point we find it is indeed a repetition (that is: if we find that our
## 16th value is equal to one of the previous 15 values), we

let i--
## decrease the i value by 1, so now i=14 and

break; }
## we immediately exit from our control loop: no need to waste time. So now we're
## back to our main loop, where i is increased by one: it gets back to 15 again,
## and again we try to find a new 16th element.

done
## If instead we complete all 15 steps of our control loop, without a break, we know that
## arr[15] is not a repetition, and we go back to our main loop. As we said i value
## is incremented by 1, and  so now it is 16. And we go forth for the 17th element generation.

done
## After we've found 60 different elements, we're done