awk 'FNR==NR{                                         ####### This condition will be TRUE only when first file is being read, because FNR will be RESET for each file but NR(Number of recoreds) value will be keep on increasing till last file read.
A[++i]=$1;                                            ####### Once above condition is TRUE then I am creating an array named A whose index is a variable named i, ++i means increse value of variable i and keep it's value same as $1's(first field's) value.
B[i]=$2;                                              ####### Creating an array named B whose index is variable i(note but not increasing the value of variable i here, to keep the same indexes for array A and B). keeping it's value to $2's value which is second field's value.
next}                                                 ####### putting next statment here to skip further all the next actions now.
{for(j=1;j<=i;j++){                                   ####### Now starting a for loop to run it till the value of variable i, which we will get variable i's final value when first file will be completly read.
if(B[j]){                                             ####### Here I am making sure array B's value is NOT NULL(because in your example at last line last field is empty so during next step doing substr I have to check this condition now.
C=C?C OFS substr($0,A[j],B[j]):substr($0,A[j],B[j])}  ####### Creating a variable named C whose value will appended each time with it's own last time value along with the current line's substring's value(Here I am using array A and array B to get the substring where obvioslu array A is for the starting position and array B denotes then length of string.
else {                                                ####### If above condition is NOT true then this else will be executed.
C=C?C OFS substr($0,A[j]):substr($0,A[j])}};          ####### create a variable named C and each time append itself with variable C with it's current line's substring's value. Here difference between the previous substring and now substring is I am not giving the till value eg--> substr(LINE, STARTING point, END Point); because we may have NO END point like your 3rd line in fields file.
print C;                                              ####### printing the variable named C.
C=""}'                                                ####### Nullyfing the variable C.
FS="," fields                                         ####### Mentioning the field seprator for fields file as comma here. NOTE it will not be for second file, awk gives us this facility to set mutiple field seprators for different files according to our requirements.
OFS="|" main_file                                     #######  Mentioning the output field seprator as | here and mentioning Input_file(main_file) here too.