As an example this
mp3.playWithVolume(int8_t index, int8_t volume);
describes a function called playWithVolume() that requires two parameters - both of the of datatype int8_t which is a signed 8 bit number and is the same as the Arduino char datatype.
The first parameter will be treated as an index (presumably to choose a song) and the second parameter will be treated as the volume at which the song will be played.
The names given to the parameters have no intrinsic significance. The author of the library could have called anything he wished.
...R