Where not exists is a command you can use in order to exclude lines in your load. Although it seems you have a mistake in the script, as Marcus also mentions, the basic idea behind it is that you'll load everything from load 2, except the lines where Key1/A is similar to Key 1 from your Trans load.
You need to do the Trans load, in order for Qlikview to know the value you don't want to load in advance.
Not exists checks previously loaded data so it is checking Key1 in table 1 against key A in table two. If you only want to return rows in table two which do not match a value of Key1 in table one without joining the tables this would work.
Note: You did not need to create a second field called KeyA, the code will do the same thing this way not exists(Key1)