What do Ctrl-J, Ctrl-M and Ctrl-I mean in Microsoft Notepad?

Take a look at this table; the underlying character code for Ctrl+M happens to be (decimal) 13, which is the same as the enter key. The same thing goes for the tab (Ctrl+I), and so on. Ctrl+G is the BEL character, used to alert a remote operator; it still works that way in some terminal programs.

This dates from early telecommunication days when those characters were used for 'out of band' signalling, because they're not printing characters.

The Wikipedia page has an interesting write up on it all.

The ctrl key was added to early computer keyboards so that users could generate control codes.

One way of generating control codes, which became popular, was to have a ctrl key, and use it like a shift key, to generate control codes from the standard alphabet keys. Since the control codes are the low 32 values of the ASCII map, they map exactly to the 32 values of the alphabet and punctuation symbols, exactly as the upper case alphabet maps to the lower case alphabet.

A few of the control codes (enter, tab, backspace, etc…) are so useful that they also have dedicated keys on the keyboard.