If you reside in California and pass away without an estate plan (a will or living trust), your estate will be considered intestate. In other words, you have left no written instructions on how you want your assets to be distributed.
As a result, your estate will have to go through the probate process, where a judge will determine the distribution of your assets based on the default guidelines outlined in the California Probate Code. This means that your assets may not be distributed according to your wishes, but rather according to the state's predetermined rules.
Also, if you have young children, and you have not left written instructions on who should raise them and be their guardian if something happens to you, a judge will choose the person to raise your children.