Explanation:

When working with file objects in Python, we have several methods available to read the contents of a file. One of these methods is `readline()`, which is used to read a single line from the file.

What is `readline()`?

The `readline()` method reads a single line from the file and returns it as a string. It starts reading from the current position of the file pointer and moves it to the next line. If the file pointer is already at the end of the file, it returns an empty string.

Usage of `readline()`:

To read the next line of a file using `readline()`, we need to follow these steps:

1. Open the file using the `open()` function and assign it to a file object (e.g., `infile`).
2. Call the `readline()` method on the file object to read the next line.

The code snippet would look like this:
```
infile = open('filename.txt', 'r')
next_line = infile.readline()
```

Here, `'filename.txt'` should be replaced with the actual filename/path of the file you want to read.

Alternative methods:

- `read(2)`: This method reads a specified number of characters from the file. The parameter `2` specifies that it should read 2 characters. It does not read the next line.
- `infile.read()`: This method reads the entire contents of the file as a single string. It does not read the next line.
- `infile.readlines()`: This method reads all the lines of the file and returns them as a list of strings. It does not read the next line.

Conclusion:

In order to read the next line of a file from a file object, we use the `readline()` method. This method reads a single line from the file and moves the file pointer to the next line. It is important to note that `readline()` only reads one line at a time, so if we want to read multiple lines, we need to call it multiple times.