Goggles

Aviator Goggles, normally worn around the forehead, are a traditional symbol for the leader of a digidestined team, usually referred to as a "Goggle Boy". Throughout each of the manga and the first four seasons of the anime, the leader of the Chosen Children would often wear goggles, as well as partner with a dragon- or reptile-type Digimon. This rule was broken in Digimon Data Squad with Marcus, who never wore any, though he did partner with the reptile-type Agumon.

Digimon Adventure V-Tamer 01
In Digimon Adventure V-Tamer 01, Taichi Yagami wore a pair of goggles. In order to motivate a frightened Biyomon, he explains in "GOGGLES..." that his grandfather gave the goggles to him after Taichi saw a friend being bullied and didn't know what to do. They had belonged to a friend of his grandfather who was brave and loved his friends, and would often say "Courage will give you wings to fly". However, the friend had died of an illness, and Taichi's grandfather had kept them to remember him. They were given to Taichi to remind him to never forget his courage, and that he must be brave to protect his friends.

Taichi also wears the goggles while flying on AeroVeedramon Zeromaru. His grandfather also calls them "Goggles of Courage and Friendship", Taichi's two main qualities.

Digimon Adventure
In Digimon Adventure, Tai wears goggles identical to his manga counterpart. However, their significance or meaning is not covered. He is also shown wearing the goggles as a child, during the flashback in "The Ultimate Clash" and the pilot film Digimon Adventure (the first segment of Digimon: The Movie).

Digimon Adventure 02
In Enter Flamedramon, Davis is introduced wearing goggles, which initially caused T.K. to confuse him with Tai. Later, after Davis and Flamedramon save Kari from a Monochromon, Tai passes his own goggles onto Davis, and eventually leads the new Digidestined.

Davis' image song is entitled "Goggle Boy".

Digimon Tamers
In Digimon Tamers, Takato wears goggles to show his status as a Tamer to Guilmon. Later on in the series, he also uses them to shield his eyes when entering Digital Fields, or during the Digital World's dust storms.

Digimon Frontier
In Digimon Frontier, Takuya wears goggles, though his reason for it is not covered. However, he does use them during a sandstorm in the Digital World.

Trivia

 * As of yet, no female has worn goggles in any Digimon series.