Hemlock poisoning is a side effect a person ingesting hemlock. The victim finds it difficult to breathe at first and then slowly falls into unconciousness, possibily leading to fatality.
Merlin used hemlock to poison Morgana after he finds out that she is the source of magic making Camelot go to sleep. He took it from Gaius's chamber and mixed it with water and offered it to Morganna. She drank the water, getting poisoned by the hemlock in the process. After Morgause heard her scream, she rushed to Morgana and then vanished, taking Morgana with her. Morgana survived this, and began to harbour a hatred for Merlin.