It uses an associative array (a hash), so it guarantees the uniqueness
of the key ($NF in this case) and the value is alway the last one it sees
(the one in file2). It will associate to every key ($NF) the entire record ($0) and it will update the value when it sees the same key.

Well, this is kinda style of writing,
it you want the code more readable,
you could use this instead (and this is compatible even with the old plain Solaris awk):

awk '{
  key_record[$NF] = $0     # associate key ($NF) with entire record ($0) 
  }
END { 
  # after the entire input has been read 
  for (key  in key_record) # for every key stored
    print key_record[key]  # print the associated value
    }' file1 file2

About the sort command wouldn't it fail on the ';' ?

Do you know how to specify 'last field' of line with sort ? Or is something like : | awk '{ printf substr($NF, 1, length($NF)-1);$NF = "";printf " %s\n",$0 }' | sort -n | awk '{ printf "%s%s;\n",$0,$1 }' | awk '{$1="";sub(/^ +/, "");printf "%s\n",$0}' preferable ?

perl -lane'
  $h{$F[-1]} = $_;
  print join "\n", map $h{$_}, sort {$a <=> $b} keys %h 
    if eof'

perl -lane'
  chop $F[-1] and $h{$F[-1]} = $_;
  print join "\n", map $h{$_}, sort {$a <=> $b} keys %h 
    if eof'

read<file;set -- $REPLY;sort -k$#n file