If you own your property as a beneficial joint tenant, this means that it belongs to you and the other owner(s) jointly. You can't re-mortgage or sell the property without the agreement of all the other owner(s). However if there is a dispute, an owner can apply for a court order.
As a beneficial joint tenant, you don't own specific shares in the property and you can't give away a share of the property in a will. If you die, your interest in the property passes automatically to the other owner(s).