There are two types

1. 24 Hours Freeleech - With these tokens you will be able to download any torrent within 24 hours after activation of the token. In this time download will not be counted. Trackers like Blutopia offers these kind of tokens.

2. Specific Torrent Freeleech - With these tokens you can mark a torrent as freeleech for you. Only one torrent will be freeleech with one token. Trackers like Redacted offers these kind of tokens.

so Freeleech token as the name suggests, is a token that you can use o make one torrent freeleech i.e. to only count your upload and not your download for that torrent.
Trackers like Blutopia, Alpharatio, Red etc have freeleech tokens that can be earned by participating in events or by doing a task for example, you get an FLToken in blutopia by seeding a dead torrent for 30 days, alpharatio gives you tokens on seeding and hanging out in IRC etc.

Trackers also have their own limitations on the usage of these tokens like alpharatio only allows torrents up to 24GB to be eligible to become freeleech through a token for four days.

These are mainly used to maintain the ratio in case you want to download something big from a tracker you just joined and the torrent you want to download is not freeleech.

another 24 hour FL token example
screenshot taken from store
https://imgur.com/O9zWbIV

Also redeem FL token from marketplace for yourself or to others.