Lots of people imagine they can gift their share of a house via their Will.
Most commonly, people who own their home jointly this would mean that on death, the property becomes solely owned by the surviving joint owner. The surviving owner is then free to do as they please.
Severance of tenancy is the legal transaction that changes the ownership into tenants in common each owning 50% and thus allowing that 50% to be disposed of in your Will as you wish. Allowing you to decide on how your share of the property is distributed.