How Computers Work - Software

Machine Code

"Software" is the general category of code which runs on the hardware. If the hardware is a player piano, then the software is the music. The common case is a "program" like Firefox -- software you run on your computer to solve a particular problem. A computer can run multiple programs at the same time, keeping their use of memory, drawing in windows etc. separated so they hopefully do not interfere with each other.
A CPU understands a low level "machine code" language (also known as "native code"). The language of the machine code is hardwired into the design of the CPU hardware; it is not something that can be changed at will. Each family of compatible CPUs (e.g. the very popular Intel x86 family) has its own, idiosyncratic machine code which is not compatible with the machine code of other CPU families.