I am new to Multicharts and want to understand following code

Vars:
value1(0)
depth(12),
shift(0);


for shift = ( currentbar - depth ) downto 0
begin
value1 = Low[LowestBar(low, depth)[shift] ];
end;

what is index of
LowestBar(low, depth)[shift]

I'm not sure what you mean by index. Here is what I understand the code to do.

for shift = ( currentbar - depth ) downto 0 begin
{For all the bars since the 12th bar on the chart}

value1 = Low[LowestBar(low, depth)[shift] ];

Write to Value1 the Low of the lowest bar based on the low for 12 bars back. Its easier to break it up.

Low[ Barnumber ] Finds the low value of the bar "Barnumber" bars back.

LowestBar (low, depth) -
A. The word low means that you want the lowest low instead of the lowest close or lowest high, etc.
B. Depth is the number of bars back you want to look to find the low. In this case 12.

That means that you would return the barnumber of the lowest low in the last 12 bars.

Shift would be the number of bars back that you want to obtain the result from. For instance, zero means that you want to know for this bar. If Shift = 1, it would find the lowest low of the 12 bars preceding the bar on bar before the current bar. The coder would generally be better off writing the data into an array than using shift. Its confusing and takes longer to process.