A holographic will is a will handwritten by the testator (the maker of a will). Holographic wills are recognized in about half of the states in the United States.
My state, Florida, recognizes holographic wills under limited circumstances. If, for example, a will was handwritten by its maker, Florida will recognize the will so long as it was executed with the formalities required under state law. Florida requires, among other things, that at least two attesting witnesses must sign the will in the presence of each other, and in the presence of the maker.
While a holographic will could be valid in Florida, there are many cases where attempts to comply with the Florida will statute failed.
If the maker of a holographic will lives in a state that does not accept holographic wills, the will is not valid.
When a Will Is Not Valid
Use of an invalid holographic can have dire consequences.
If there are not any valid prior wills, the testator's assets will pass to the heirs according to state law, not the intent of the testator. This is called intestacy. It is problematic for someone who makes a will — handwritten or not, because the will maker intended for the assets to pass in a particular way, and certainly not how the state's legislature deems best.
In my experience, some people are not overly concerned with making a will. That sentiment could be caused by an individual not having many assets, or the individual relying on will alternatives, like intestacy laws, joint bank accounts, payable or transfer on death accounts, and certain deeds, such as real property owned tenancy by entirety (property owned by a married couple) or a lady bird deed.
The latter, which is also known as an enhanced life estate deed, allows the owner of the real property to have control of the property until the owner dies. The property is then transferred automatically to the person(s) named in the deed. You could think of a lady bird deed as being a pay-on-death account for real estate.