Samudaya refers to the root cause of suffering, as described by the Buddha in his first sermon; the 4 Noble Truths.
The 4 Noble Truths are:
1) Dukkha – Suffering and a lingering sense of unfulfilment, which is inherent in all realms of existence
2) Samudaya – The Cause of Suffering, which arises from craving, attachment and ignorance
3) Nirodha – Cessation of Suffering, where one cuts off the root of suffering by achieving liberation from the cycle of death and rebirth, thereby entering Nibbana
4) Magga – The Path leading to the transcendence of suffering, i.e. the Noble Eightfold path