char
in C/C++ is a signed 1-byte integer. This GUI is simply expressing that value as a signed decimal number, and the equivalent ASCII character.
You're asking about the byte value -1, which can be interpretted in the following ways:
Binary 11111111
Octal 0377
Hex 0xFF
Decimal -1 (Signed)
255 (Unsigned)
ASCII \377
\xFF
Note that there isn't a standard printable ASCII character for 255, which is why they show it like they do.
Another Example:
Binary 01001010
Octal 0112
Hex 0x4A
Decimal 74 (Signed)
74 (Unsigned)
ASCII 'J'