An open mortgage gives you a lot of flexibility to increase your mortgage payments, either as a lump sum or by increasing your regular payments. This lets you pay down your mortgage faster, or pay the whole thing off quickly. You might use this feature if you sold your home or received a large sum of money, such as from an inheritance.
The opposite of an open mortgage is a closed mortgage. Closed mortgages limit the amount by which you can increase your payments, and will charge you penalties if you choose to do so. While there's less flexibility in closed mortgages, you will pay a lower rate on a closed mortgage than an open mortgage.
Open mortgages are less common than closed mortgages in Canada. This is because open mortgages come with higher rates, which are only worth paying in specific, and reasonably uncommon, situations.