"Stacked Achievements" refers to the achievements system on the xbox 360. All 360 games have certain goals or challenges that when performed earn achievements. Each achievement adds to your gamerscore. A stacked achievement works like this: Say you have 8 chapters in a game and each chapter beaten earns a 10 point achievement. Chapter 1 beaten on easy would be 10 points, 20 points on medium, and 30 points on hard. If you beat Chapter 1 on hard your first time playing, you would unlock all 3 achievements for easy medium and hard. This would be a total of 60 points, and is called "stacked" because you earn all 3 without having to play through Chapter 1 3 times, once on each difficulty. Not all 360 games offer stacked Achievements, you can search the internet for games that do. Some examples include Gears of War and Halo 3.
"I beat Halo 3 on Legendary (very hard) my first time playing and got all the stacked achievements."
"Drop in, drop out" usually refers to online video game playing where any player wishing to join the game in progress can "drop in" and assume instant control of a NPC (computer controlled player), without any significant disruption to the gameplay. The player can also leave and return control of the character to the computer - "drop out" again without causing disruption to the game.
Gears of War offers drop in, drop out gameplay where any player can assume control of Dominic Santiago, Marcus Fenix's best friend, without freezing or restarting the level. World of Warcraft also offers this feature.