Cardamom ‘Look Gra Wan’ in Thai, is the common name for a group of distinct plants, members of the ginger family. As opposed to other edible gingers, it is the seeds within the fruit pods that are used in cooking, rather than the rhizomes.
The most common variety found in Thailand is the white cardamom. It is the fruit of a plant called Amomum Krervanh Pierre. The pod of white cardamom is quite round compared to other varieties and has a light brown or beige tint. The flavor is considered milder than than that of the green cardamom, although I find the camphor profile of white cardamom to be sharper than that of the green variety. Another variety used in Thailand is the black cardamom, it has larger black pods and a smoky scent.
It is likely that cardamom has been introduced to the Thai cuisine through Muslim influences. While it hasn’t been adopted throughout, it is still noteworthy due to the role it plays in some of the most famous curries. White is used for instance in Gaeng Massaman while black cardamom is used in Gaeng Khao Soi, in both it is dry roasted, pound into a powder and then added to the curry pastes. If you cannot find white cardamom, you may replace it with the green variety and would probably need to reduce the quantity a bit.