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 reccomended over soft resetting if soft resetting is possible.