OS module in Python provides functions for interacting with the operating system. OS comes under Python’s standard utility modules. This module provides a portable way of using operating system dependent functionality. os.path module is sub-module of OS module in Python used for common pathname manipulation.

os.path.join() is a method in Python's os module that is used to join one or more path components intelligently, regardless of the operating system. This method takes one or more path components and joins them using the correct path separator for the operating system the code is running on.

The method accepts multiple arguments, each of which represents a directory or file name. It then returns a new string that represents the concatenation of the paths with the appropriate separator between them. For example, on Unix-like systems, the separator is "/", while on Windows systems, it is "\".

Here is an example of how to use os.path.join() method: 

import os

# Join two path components using os.path.join()
path1 = '/home/user'
path2 = 'Documents'
full_path = os.path.join(path1, path2)

# Print the full path
print(full_path)

In this example, os.path.join() method is used to concatenate two path components, /home/user and Documents, into a single path '/home/user/Documents'. The method ensures that the correct separator is used between the two path components.

Using os.path.join() method to construct file paths in Python is highly recommended as it ensures that the code works correctly across different operating systems.