When you "append" something to a text object, it will add what you want after the current text.
If you have a text object with "Hello" written on it and append " world" after it (with the space), you would get "Hello world".
If you have a text object with "Hello" written on it and used SET TEXT with " world", the text would erase and become " world".
The sprite IID is the instance ID that it has when it's created. If you notice, all the sprites with numers are the same sprite, just with different animations. When the game starts it loads these sprites one by one, and each one has a single IID, from 0 to 3. The str() expression converts this number into a string, and the append adds it to the text object.