Hello Zouhair,

Cr means a "cursor"  which is the concept of SQL. The role of cursor(cr in Odoo) is to enumerate/iterate over each rows in result set one by one.
Hence cursor  act like a looping statement to fetch  data row by row basis

Enough of theory now , lets take an simple example of sql:

import psycopg2

conn = psycopg2.connect(database="mydb", user="postgres", password="postgres", host="127.0.0.1", port="5432")

cur = conn.cursor()  #cursor object is created

cur.execute('''select name,date_order from sale.order''')

rows = cur.fetchall()

cur.close()