Mana is a resource consumed by the player in order to use magic weapons. The player's current and maximum Mana is shown as a horizontal meter of blue stars on the right side of the screen, just under the Health bar.
Each star represents 20 mana. As it drains, pieces of the stars will empty into quarter-star pieces that represent 5 mana each.
The player's maximum mana can exceed 400, the limit previously set by Terraria.