To reset a game, in general, is to have it start back at where it would be if it were just loaded by the game system (in some cases just when the game system is turned on. Resetting is sometimes necessary to execute certain glitches, such as the Surfing glitch. Resetting is also often necessary during a game freeze. There are two kinds of resetting in general: soft resetting and hard resetting.
To soft reset is to reset a game with an in-game method, such as a button combination. This is often considered safer than hard resetting, though it is not always
possible to do.
Hard resetting, in short, is turning off the game system and then turning it back on. It is often used when soft resetting is not possible, such as a game freeze. It is not recommended over soft resetting if soft resetting is possible.
On a Game Boy or Game Boy Advance system, hard resetting will also load the initial boot screen which makes the reset slightly longer.
In speedrunning a hard reset exploit involving the Game Boy Color bootstrap ROM is used for luck manipulation. The exploit is not emulated correctly for most emulators except for the latest version of BGB.