Imagine your house is like an apple.
If you own your house as joint tenants, then you both hold this red apple together as a whole. If one of you dies,then the apple passes in its entirety to the survivor automatically, so the survivor ends up owning the whole apple on their own. This is the default position for the co-ownership of land.
BUT... you can "sever" the joint tenancy over your property to become tenants in common. This is like cutting the apple in half.
Once the apple has been cut in half, you will then own a distinct one-half share of the apple each. This means that, on your death, your respective halves of the apple pass under your Will to the beneficiaries you name in your Will. It also means that, whilst you can give your half of the apple to the survivor of you (in which event, he or she would end up holding the whole apple, except it will be in two halves), you may prefer to give your half of the apple to someone else – for example, your children.
The other benefit of owning as tenants in common, is that you do not have to cut the apple into equal one-half shares; you could have unequal portions – perhaps to take into account unequal contributions to the purchase price.