A Pendulum Scale is a set of indicators unique to Pendulum Monsters. They are located above the "Monster Effect" text box of the Pendulum Monster, with the "Pendulum Effect" of the Pendulum Monster placed between them. Each Pendulum Monster has a Pendulum Scale value, signified by the blue symbol and the red symbol.
Pendulum Scales are used in Pendulum Summoning. You can Pendulum Summon monsters from the hand and/or Pendulum Monsters that have been sent to the Extra Deck whose Levels are in between the Pendulum Scale values.
It is currently unknown whether a monster can have two different Pendulum Scale values.