Rama is a title given to Thai kings of the currently ruling Chakri dynasty. As there have so far been nine kings, each can be called Rama I, II etc, up to the current Rama IX.
The practice of calling the kings Rama was adopted by Vajiravudh, also known as Rama VI, who was aware that farang - foreigners - had difficulty pronouncing the long honorific titles that most Thai use to refer to their monarchs. Why Rama? After the hero of the pan-Asian epic, the Ramayana, or Ramakien, as it is known in Thai.
All the Ramas so far: