characters are really integer values. You can add them together. If you look at an ascii chart, you'll see that 'a' is equal to 97, and 'b' is 98. so when you add ch + 'b', you effectively have 97 + 98, or 195.
When you then try and print this, it is outside the normal printable range, so you can get something weird.
There are only two hard things in computer science: cache invalidation, naming things, and off-by-one errors
The console (cmd.exe on Windows, bash / csh / ... on Linux) is very limited in the characters it can display. Use JOptionPane to show the chars will be more informative, although it still can't display all characters:
You will have to follow Fred's suggestion of getting an ASCII chart, or a basic Unicode chart, which has the same values. Add the two chars together (remembering Unicode gives their values in hexadecimal, so use the hex option on your computer's calculator), and compare them with the ASCII values or the "extended" Unicode chart. Then you can see what the sum is supposed to mean. If you go beyond those two charts, the Unicode site will give lots more characters.
Remember that the DOS and Windows® command lines only have a restricted range of characters they can display, so even something like £ comes out as ú on it. And many characters simply come out as boxes or ?