The unicode_start command will put the keyboard and console into Unicode (UTF-8) mode.
For the keyboard this means that one can attach 16-bit U+xxxx values to keyboard keys using loadkeys(1), and have these appear as UTF-8 input to user programs. Also, that one can type hexadecimal Alt-xxxx using the numeric keypad, and again produce UTF-8.
For the console this means that the kernel expects UTF-8 output from user programs, and displays the output accordingly.
The parameter font is a font that is loaded. It should have a built-in Unicode map, or, if it hasn't, such a map can be given explicitly as second parameter. When no font was specified, the current font is kept.