"What is this line?"
Aaaaaaah, to understand that right-hand text limit (as it is called in the MATLAB settings) you have to delve into the history of computers. In short, it is because a long time ago computers could only hold/process/print a limited number of characters per line (limited by various technologies):
You can find more detailed history of this line online.
Programmers hold onto that line like a kind of comfort blanket, even though any technical neccessity it might once have had has been irrelevant for more than half a century. Feel free to ignore the line if you want (or change its placement in the settings), with the understanding that programmers are very delicate creatures and are easily surprised by small changes in what they see (in your code, in other languages, etc) and will let you know that you are doing something very very wrong... but are not really able to state exactly what.
Perhaps some of them will reply here.
