A token is a song that unlocks on certain conditions.
Normal Token[]
A normal token, also called an achievement & event token, is unlocked by achieving certain milestones in the game such as 1 hour playtime or is given in officially sanctioned events. A full list can be found here. Achievement tokens are unlocked only after visiting the levelranks page.
In the past, normal tokens were scattered throughout the FFR site which had to be found to unlock. These were replaced by achievement tokens. Their descriptions can be found here.
Skill Token[]
A skill token is unlocked by meeting a certain condition on a song or a set of songs. Some skill tokens require scoring well on certain songs, while some skill tokens have anti-skill requirements where players have to pull off an odd requirement. For example, 0MGWTFT0K3N requires playing Excite Bike with 2 perfects, 4 goods, 6 averages, 8 misses, 10 boos, and 12 max combo. A full list can be found here.