It can refer to:
* Senna plants: the genus *Cassia*, which includes several plants used in traditional medicine
* Senna leaves: dried leaves of these plants, used as a laxative
* Senna tea: a herbal tea made from senna leaves
However, "kasja" can also mean "cinnamon" in Polish. So, to be clear about which meaning is intended, it's important to use the full term, such as "kasja seneska" (senna plant) or "herbata z kasji seneskiej" (senna tea).