A Rank, like a Level, is a rough indication of how powerful a monster is. The higher the Rank, the stronger it is. Ranks are primarily used with Xyz and Nova Monsters, but some Hybrid Monsters (those who specifically state Xyz Monsters as their Material) can also have Ranks. They are displayed underneath the first text box (Name/Attribute) on a Monster Card, on the left-hand side. A Rank is depicted as a yellow star on a black circle. The Rank of an Xyz Monster is the same the Level of its Xyz Materials, and the Rank of a Nova Monster is the same as the Level of its Synchro counterpart.
Although Ranks are a lot like Levels, they are completely different, and any card that refers to or requires either of them do not apply to the other.
E.g.only affects monsters with Levels, not Ranks.
E.g. To Synchro Summon a Synchro Monster, you must have monsters whose Levels total the Synchro Monster's Level, so you cannot use monsters with Ranks instead.