Psyllium husk is the water-absorbing outer covering of seeds from certain Plantago species. In baking, its gel helps bind water and gives gluten-free dough cohesion and flexibility. It can be sold as whole husks or a finely milled powder.
Those forms hydrate differently and are not always interchangeable by volume. Use the form and liquid quantities specified in a tested recipe. It does not supply gluten and is not a direct one-for-one replacement for flour.
Reference: Further reading.