In order to gain access to a file, a computer program needs to interact with the operating system; the process is called file I/O. File I/O is the way in which a programming language is able to open, read from, write to, and close files. (The I/O here stands for input/output, because the program takes input from certain files and writes output to other files.)
While it is not always necessary to explicitly open a file, it is important to conceptually separate the process of opening a file from the process of reading the contents of a file. This is because it is often necessary to read the contents of a file incrementally or specify particular parameters when the file is opened.
In the following subsection, I will show to use python to open a file.