If you die without leaving a Will you are said to have died intestate.
Without a Will a set of statutory rules (the laws of Intestacy) are imposed which specify how assets are distributed to family members in a fixed order.
If you have no family members then your assets will go to the Crown.
The danger of dying intestate is that you will have no say in who gets what. Family members who you may not wish to could inherit your assets. Likewise, people who are not blood relatives, such as unmarried partners, may not receive anything.
It is critical that you make a Will to avoid this situation.