A holograph will (or holographic will) is a will that the testator (the person making the will) themselves writes entirely by hand and signs.
In England and Wales, a holograph will, like any other will, is valid only if it is properly signed and witnessed. If unwitnessed, a holograph will is not valid, unless it is counted as a privileged will.
(A privileged will is one that is made by an individual in active military service, as the will can be made and validly executed quickly. It can be either written or 'nuncupative' – which means that it is spoken in front of a credible witness.)
The seeming attraction of a handwritten (holograph) will is that it can be drafted immediately, and would not depend on being able to meet a solicitor, or having access to a computer or the internet. A deathbed will might theoretically fall into that category.
But the risks and problems can outweigh the benefits. Although you do not have to be legally trained to write your will, it is much more sensible to equip yourself with the right tools for the job. One of those tools might be professional advice; another might be to use a will-writing service that is run with professional supervision.
Writing your will by hand on your own:
It is worth using a recognised service to avoid all these pitfalls and ensure that your will does the right job.