Tendai, also known as Japanese Tiantai, is a school of Buddhism that was founded by the monk Saicho in Japan during the 9th century. It is based on the teachings of the Chinese Tiantai school, which was founded by the monk Zhiyi in the 6th century.

The main teachings of Tendai can be summarized by the concept of "three truths" or "threefold truth." These three truths are the ultimate truth, the conventional truth, and the middle way.

The ultimate truth refers to the idea that all things are ultimately empty and interconnected. This is based on the Buddhist concept of emptiness, which states that all phenomena are devoid of inherent existence. In Tendai, this ultimate truth is also seen as the ultimate reality or the true nature of all things.

The conventional truth, on the other hand, refers to the world of appearances and phenomena that we experience in our daily lives. This includes our thoughts, emotions, and physical surroundings. Tendai teaches that the conventional truth is not separate from the ultimate truth, but rather a manifestation of it.

The middle way is the path that leads to the realization of the ultimate truth. It is the balance between the extreme views of existence and non-existence, and the understanding that both the ultimate and conventional truths are equally important.

Another important teaching of Tendai is the doctrine of "three thousand realms in a single moment of thought." This concept states that within each moment of our thoughts, there are three thousand realms or possibilities. This means that every thought has the potential to create positive or negative outcomes, and it is up to us to choose which path to take.

Tendai also emphasizes the importance of the Lotus Sutra, a central text in Mahayana Buddhism. The school teaches that the Lotus Sutra contains the ultimate truth and is the key to attaining enlightenment.

In addition to these main teachings, Tendai also incorporates various practices such as meditation, chanting, and ritual ceremonies. These practices are meant to help practitioners cultivate wisdom and compassion, and ultimately reach enlightenment.

In conclusion, the main teachings of Tendai revolve around the concept of three truths, the importance of the Lotus Sutra, and the practice of meditation and other rituals. By understanding and embodying these teachings, followers of Tendai aim to achieve a deep understanding of the ultimate truth and attain enlightenment.