__LINE__ returns the line of code its on, and % 10 means “remainder 10.” Examples:

1 % 10 == 1
...
8 % 10 == 8
9 % 10 == 9
10 % 10 == 0 <-- loops back to 0
11 % 10 == 1
12 % 10 == 2
...
19 % 10 == 9
20 % 10 == 0
21 % 10 == 1

In code, 0 means false and 1 (and 2, 3, 4, …) means true.

So, if on line 10, you say:

int dont_delete_database = true;

then it will expand to:

int dont_delete_database = ( 10 % 10 );
// 10 % 10 == 0 which means false
// database dies...

if you add a line before it, so that the code moves to line 11, then suddenly it works:

// THIS COMMENT PREVENTS DATABASE FROM DYING
int dont_delete_database = ( 11 % 10 );
// 11 % 10 == 1, which means true