SOP is short for statement of purpose. It's also known as study plan.
It is an essay that you attach with your application in order to state everything about yourself and your plans ahead, it's like a substitute for an interview. You should include the following in it
Why do you wish to study in Canada in the program for which you have been accepted?
What is your overall educational goal?
Why are you not pursuing a similar program in your country of residence/citizenship?
What research have you done into studies in your country of residence/citizenship?
How will this program enhance your employment opportunities in your country of residence/citizenship?
What ties do you have to your country of residence/citizenship?
If it has been more than 2 years since you have left full-time education, please provide your C.V. (resumé).
In the case of a minor applicant, what are your reasons for studying in Canada? What is your parents’/guardians’ immigration status in their current country of residence?
Provide details of your education history – dates when the course started and ended, the name and address of the school, the course taken, qualification/degree/certificate awarded for the course