Core are colored gems that are used in Battle Spirits games. Their main function is to act as a resource for summoning and maintaining Spirits/Ultimates/Braves, deploying and upgrading Nexuses, and the usage of Spells. Core is used to keep track of your Life as well.
Players start with 5 life and 4 reserve cores -- as of April 1, 2015, one of the reserve core will be a Soul Core. Each turn (except for the leading player's first turn), players may take one core from the Void (a pool of unused core) during the Core Step.
Core cannot be altered in your Life except as a result of an effect or an opposing attack, in which case the Life core deducted is sent to your Reserve (except if effects dictate another location).
Another type of core, Soul Core, has been introduced. Soul Core is used much the same as regular core, however there are specific effects that only allow the usage of Soul Core.