Like it's predecessor, Metroid, Metroid 2 is a platformer which takes place in a large interconnected world. However, this time around progression through the game has less to do with items (although they do play a role). Progression is determined mainly by defeating Metroids, which somehow causes the acid in the planet to lower.
At the bottom of the screen your current amounts of energy and missiles are shown. At the very right the Metroid detector, which shows how many Metroids are left in the game. Pausing the game reveals the "L" counter, which shows how many Metroids are left in the area.