Tokens are Grey Framed Game Pieces that represent as Monsters on the field and disappear out of existence when it leaves.
Tokens cannot be summoned by a player at any time without reason; can only be summoned as a result of a monster's effect that allows a token to be summoned.
There are a few rules that Token must follow and it is recommended for those who are new to Tokens or deal with them (Credits to Yu-Gi-Oh! Wikia)
• Tokens cannot be facedown and cannot be forced facedown especially with Book of Moon, Maestroke the Symphony Djinn, etc. However a card that does not target and changes monsters to facedown positions will change Tokens to face-up defense instead (i.e. Swords of Concealing Light)
• Tokens can be returned to the hand or deck by a card effect, but not for a cost (i.e. Ally of Justice Birdman/ Blackwing - Zephyros the Elite)
• Tokens can be tributed, if the card that summons them allows it
• Tokens can be used for a synchro summon, if the card that summons them allows it
• Tokens are always Normal Monsters
• Tokens cannot be used for Xyz Summons
• Tokens can be used as a Fusion Material, but cannot be sent to the graveyard for the Summon of Chimeratech Fortress Dragon
• Token monsters can be used as Equip Cards if and only if they weren't removed from the field in the process.
• Tokens cannot exist anywhere else but the field.