It is a clipboard function. It will appeand what every you have selected to what every is in the clip board.
So you have the following in a query window:
select * from some_table
If select the word "from" and say copy this goes to the clipboard. If I then select "some_table" and select append it will be appended to the end of the clipboard so now the clipboard will have "fromsome_table" in it. so if you need spaces between make sure you are selecting a space before what you want to append.