The code you have provided is for connecting to a MySQL database and querying the database.

The first step is to import the necessary libraries. In this case, we need to import the decouple library, which is used to read environment variables, and the mysql.connector library, which is used to connect to MySQL databases.

The next step is to define the connection parameters. These parameters include the username, password, host, database name, and port number.

Once the connection parameters have been defined, we can connect to the database. To do this, we use the connect() method of the mysql.connector library.

The connect() method takes the connection parameters as arguments. It returns a connection object, which we can use to execute queries.

To execute a query, we use the execute() method of the connection object. The execute() method takes the query as an argument.

The execute() method returns a cursor object, which we can use to iterate over the results of the query.

To iterate over the results of the query, we use the fetchall() method of the cursor object. The fetchall() method returns a list of tuples, where each tuple represents a row in the result set.

The following code shows how to connect to a MySQL database and query the database:

import decouple
import mysql.connector

# Define the connection parameters.
user = decouple.config('MYSQL_USER')
password = decouple.config('MYSQL_PASSWORD')
host = decouple.config('MYSQL_HOST')
database = decouple.config('MYSQL_DB')
port = decouple.config('MYSQL_PORT')

# Connect to the database.
connection = mysql.connector.connect(user=user, password=password, host=host, database=database, port=port)

# Create a cursor object.
cursor = connection.cursor()

# Execute a query.
cursor.execute("SELECT name, countrycode FROM city")

# Iterate over the results of the query.
for name, countrycode in cursor.fetchall():
    print(name, "|", countrycode)

# Close the connection.
connection.close()

I hope this helps!