Game play is the involvement within the game. Basically it's how you go about playing the game with the character you got. They are different types of ways of going about game play like: Strategy, turn-based, simulation and MMORPG are pretty much the main ones.
Strategy is based on the outcome of the players decision. The character is control ed and therefore the player itself.
Turn-based is when you and the enemy take it in turns to attack.
Simulation is a real life situation. A lot of simulation games come from board games such as Monopoly and Risk. It's a combination of strategy and luck.
MMORPG is a lot like strategy except for one tinsy detail: you can play against other people from all over the world which makes games such as: (Sorry Mike) World of warcraft, Runescape and one-person shooters like Gears of war and Halo popular.
Game play is important in itself because that's the whole point of playing a game. Without game play then there's no point. In a way game play is like telling a story, even though there's a set story for the character and you have to do certain things in order to proceed there's always the players story to tell because their the ones making it happen based on their own decisions.