awk -v RP="$var" '                          #### defining the variable named var into awk variable named RP by defining it with -v RP, if we have to get shell variable's values into awk we have to do this procedure, we can't directly get shell variable's value into awk variables.
BEGIN                                       #### starting BEGIN block/section here, which is one of awk's block where statements will be executed first, before reading files or doing other operations.
{for (n=split(RP, T, "[\n:]"); n>0; n-=2)   #### starting a for loop here, whose starting value is variable n(whose value we are getting by the number of elements in array T which is defined by split, split is an awk's built-in utility where we could take any variable's or line's value into array by defining delimiter. eg--> split(line/vriable, array_name,delimiter)). Like here we are splittig variable named rp and storing it's values to array named T and delimiter is either new_line or colon(:). Then defining the condition as per for loop's syntax eg--> for(variable_initilization, condition,decrement/increment of variable).
REPL[T[n-1]] = T[n]                         #### Here we are creating an array named REPL whose index is value of array T whose index is n-1. REPL  array has value of array T whose index is n here. So basically in REPL array we are storing the indexes of array REPL which we have to search in your Input_file and values of array REPL are the values which we have to get into Input_file.
}                                           #### completing the BEGIN section here.
        {for (r in REPL) sub (r, REPL[r])   #### Now starting a for loop which will traverse inside array REPL and then using sub which is an awk's internal utility to perform substitution whose syntax is eg--> sub(pattern/value which needs to be subsituted,pattern/value new one,line/variable). So here we are mentioning r,REPL[r] which means index pr array REPL in Input_file if it's present in Input_file should be subsituted with value of array REPL.
        }
1                                           #### awk works on method of pattern(condition)/action, so if any condition is TRUE it performes the action mentioned after it. So here by mentioning 1 we are making the condition TRUE and didn't mention any action here so it performs by default action here which is printing the current line.
' file                                      #### mentioning Input_file here.