The essential duties of your position, also known as Bona Fide Occupational Requirements (BFOR), are the main tasks that are required of you in your job. They are crucial and central to your job. They are not mere preferences, but rather indispensable requirements that must be performed.
The Meiorin* decision sets out the steps to establish what tasks should be considered BFORs:
(1) [has] the employer adopted the standard for a purpose rationally connected to the performance of the job?
(2) [has] the employer adopted the particular standard in an honest and good faith belief that it was necessary to the fulfilment of that legitimate work-related purpose? and
(3) [Is] the standard reasonably necessary to the accomplishment of that legitimate work-related purpose? To show that the standard is reasonably necessary, it must be demonstrated that it is impossible to accommodate individual employees sharing the characteristics of the claimant without imposing undue hardship upon the employer.
* British Columbia (Public Service Employee Relations Commission) v. BCGSEU,  3 S.C.R. 3