termio is the name of the old System V terminal driver interface. This interface defined a termio structure used to store terminal settings, and a range of ioctl(2) operations to get and set terminal attributes.
The termio interface is now obsolete: POSIX.1-1990 standardized a modified version of this interface, under the name termios. The POSIX.1 data structure differs slightly from the System V version, and POSIX.1 defined a suite of functions to replace the various ioctl(2) operations that existed in System V. (This was done because ioctl(2) was unstandardized, and its variadic third argument does not allow argument type checking.)
If you're looking for a page called "termio", then you can probably find most of the information that you seek in either termios(3) or ioctl_tty(2).