subprocess nedir dersek?
nette şöyle bir acıklama buldum.The subprocess module allows you to spawn new processes, connect to their input/output/error pipes, and obtain their return codes. This module intends to replace several older modules and functions.
bi de soyle bi sey buldum:
A running program is called a process. Each process has its own system state, which includes memory, lists of open files, a program counter that keeps track of the instruction being executed, and a call stack used to hold the local variables of functions.
Normally, a process executes statements one after the other in a single sequence of control flow, which is sometimes called the main thread of the process. At any given time, the program is only doing one thing.
A program can create new processes using library functions such as those found in the os or subprocess modules such as os.fork(), subprocess.Popen(), etc. However, these processes, known as subprocesses, run as completely independent entities-each with their own private system state and main thread of execution.
Because a subprocess is independent, it executes concurrently with the original process. That is, the process that created the subprocess can go on to work on other things while the subprocess carries out its own work behind the scenes.

peki pipe nedir dersek?
Unix or Linux without pipes is unthinkable, or at least, pipelines are a very important part of Unix and Linux applications. Small elements are put together by using pipes. Processes are chained together by their standard streams, i.e. the output of one process is used as the input of another process. To chain processes like this, so-called anonomymous pipes are used.