Small pieces of chicken are marinated, put on a small skewer and grilled for about 5 minutes.
Traditionally, yakitori has just meant chicken on a skewer, with perhaps some vegetable such as green onion as well, but now the term is being used, even in Japan, for fish, veg, beef, pork, seafood, etc cooked on a skewer.
Bamboo skewers are traditionally used; they need to be soaked in water first so that they don’t burn.
In Japanese, “yaki” means “grilled”. “Tori” means “poultry”.