A starter Pokémon is one of a trio of Grass, Fire, and Water type Pokémon, usually at level 5, offered by a professor at the start of each main game, from which the player picks one and the rival picks another. The trio follows the pattern Grass beats Water, Water beats Fire, and Fire beats Grass.
The choice is the first real decision of every game. Kanto offers Bulbasaur, Charmander, and Squirtle. Every region since has followed with its own trio, ending at Sprigatito, Fuecoco, and Quaxly in Paldea. Starters are three stage lines with high, even stat growth, designed to stay useful for the whole game, and their final evolutions often gain a second type, such as Empoleon becoming Water and Steel or Incineroar becoming Fire and Dark.
Starters are also deliberately scarce. You cannot catch another player's starter species in the wild in most games, which made trading between versions the only way to complete the set, and events or later features such as the Friend Safari and Hidden Grottos only occasionally loosened that. Their hidden Abilities and egg moves are locked behind breeding and distribution for the same reason. In Pokémon Yellow and Let's Go, Pikachu and Eevee break the trio format as fixed partners instead.
Which starter you pick shapes the early game more than the late game. Charmander struggles through Kanto's first two Gyms while Bulbasaur cruises, then the balance flips. Experienced players pick for the final evolution or the speedrun route; most players pick the one they like looking at for forty hours, which is also a valid system.
In play"I always pick the Fire starter and then suffer through the first gym like an idiot."