Rally Banner

From Destinypedia, the Destiny wiki

Rally Banners are banners that can be placed at certain locations, typically before the beginning of a Raid encounter. Interacting with the banner once it is placed will grant the player full ammunition to all weapons as well as recharging all abilities and Supers.

They can be purchased for Glimmer or Legendary Shards at Suraya Hawthorne in the Tower (Destiny 2).