What does the code and what does it do, if you could explain as Im a newbie to all this scipt writing.

Cheers

..

It tells awk to treat an empty line as a record separator (RS),
a line-feed (\n) as a field-separator (FS) and two line-feeds
as an output field separator (OFS).

Although you should learn how to use RS/FS/OFS -- (as pointed out up-thread) here's a down and dirty way to get around setting RS/FS/OFS (use defaults) -- this is untested and assumes there is only one blank line between groups of records, like this ...

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etc
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BEGIN {
while(getline <"data.txt" >0){
if($0 == ""){print sum; sum = 0}
sum+=$0
}
#print sum
#^^^^^^^^
# uncomment this line if the last line in your
# data file is not a blank line.
close("data.txt")
}

It might also be important to point out that AWK is not a one-size-fits-all language nor is it intended to be written once and played everywhere. Each distribution (awk, mawk, gawk, tawk etc) and each platform (linux, unix, win, mac) all have their own personalities and behaviours. It makes it easier to answer an AWK question when the distribution is known. Just a heads up.