Doki Doki is an onomatopoeic expression in Japanese that represents the sound of a beating heart and is commonly used in Japanese media, especially in anime and manga. It is often associated with the excitement and thrill of love, romance, or fear. The phrase has become a symbol of youth and excitement, capturing the essence of what it means to be alive and experiencing life's ups and downs. Doki Doki has gained even more popularity worldwide due to the success of the indie game "Doki Doki Literature Club," which uses the phrase as part of its title.