.Text is getting the text out of the textbox object.
len(x) gets the length of the text.
left(X,Y) gets the left most text from X, up to Y characters.
The text you want to show is in SourceText.Text.
We want to find out how much text is already in the destination and get one more character: len(DestText.Text) + 1
We get that many characters from the source: left(SourceText.text, len(DestText.Text) + 1)
If you walk through a couple of cycles, to start there is no text in the destination, so len(DestText.Text) = 0. We add one, and we want to get 1 character from the left of the source string: "N"
The next iteration, the (len(DestText.Text) = 1. Add 1 and we get the 2 left most characters from the source: "Ne"
The action is based on the fact that we need one more character from the source than the destination already has.