"Concatenation" is a programming term meaning "the assembling of two or more strings into a larger string" (my definition; you should look it up). In the line of code in question you are attempting to concatenate three strings: those stored in (represented by) the variables "$row_id", "$prefix", and the constant "_torrent". These sections of the manual should help you understand how to do that:
Strings
String Operators
Having the "." inside quotes makes it a literal part of a string, not an operator, and the quotes serve no purpose anyway.
Assignment ("=") is done right-to-left, so the statement is backward.
Here's the way to do it:
$row['id'] = $row_id . $prefix . _torrent;
BTW, it's common practice to use all-uppercase for constant names. Learning and following common practice makes for easier to read, thus easier to debug and modify, code.
hth