Firstly, you have to understand what a "no-contest" clause is, and what it isn't.
A no-contest clause does not prevent your Will from being challenged, particularly from somebody who has been left out of the Will. If somebody has a legitimate claim on your estate, then simply including a clause in the Will saying that nobody can challenge the Will is not legally binding, and the Will can certainly still be contested.
There are specific reasons why a Will can be challenged. These include the testator (person for whom the Will is written) not having the mental capacity to prepare the Will. Or they may have been unduly influenced, or there may have been something untoward going on with the signing process. Or the Will may have excluded a spouse, minor child, or dependent (note that in most Provinces, there is no requirement to include adult children in your Will, with the possible exception of British Columbia).
For a number of reasons a Will can be challenged, and then it would be in the hands of a judge to determine whether this claim has merit. Simply writing within the Will that it cannot be challenged does nothing. Spouses, minor children and dependents have a right to challenge a Will if they have been excluded.
Otherwise, a 95 year old man could be influenced to update his Will and leave everything to his nurse, disinheriting his entire family. If the Will included a clause to say that nobody can challenge the Will, the clause legally does nothing.
So what is a no-contest clause?
A no contest clause typically states that if a beneficiary challenges the Will in the hope of receiving a larger inheritance, then they forfeit the bequest that they have received. It would say something like
“If any beneficiary under this will seeks to obtain in any proceeding in any court an adjudication that this will or any of its provisions is void, or seeks otherwise to void, nullify, or set aside this will or any of its provisions, then the right of that person to take any interest given to him or her by this will shall be determined as it would have been determined had such person predeceased the execution of this will without issue.”
We have included this clause in all of our Wills, but please be aware that it does not mean that your Will cannot be challenged. It just discourages people from making a challenge to the Will in the hope of receiving a larger inheritance.