The small button just forward of the trigger group is the magazine release button.
The magazine release is a button or paddle-style lever on a weapon that, when depressed, releases the magazine from the weapon.
Magazines do not always drop free upon pressing the magazine release (due either to the weapon's design, the magazine's design, or both); the magazine may need to be pulled out manually from the Magazine well when the magazine release is depressed.
Examples
Button
Button magazine release is the quickest to operate, most ergonomic to the user although not ambidextrous and can be operated with one hand. The AR-15, Browning Hi-Power, M1911 are examples that use button magazine release.
Lever/Paddle
Lever/paddle magazine release, although ambidextrous require the users hand inserting the magazine into the weapon to use it. The FN FAL, AK-47 and H&K G3 are examples that use lever/paddle magazine release.
Heel
Heel magazine release are usually found on pistols. Although ergonomic for both left/right handed users, they have a disadvantage of interfering using high capacity magazines that extend from the pistol grip as they hold the (limited supply) magazine in the pistol grip with the floor plate.
Slide
Works similar to the Lever/Paddle release but slides. It can also offer ambidextrous usage to both left/right handed shooters. The FN P90 is an example that uses a sliding magazine release.
