Well, Sp. Attack and Sp, Defense are short for. Special Attack and Special Defense.
Each Pokemon has 2 attack type and 2 defense.
Pokemon like Machop have a high Attack, which helps with moves like Low kick.
Pokemon like Espeon have a high Sp. Attack, which helps with moves like Psychic.
When Machop attacks it does more damage if the opponent has a LOW defense.
When Espeon attacks it does more damage if the opponent has a LOW Sp. Defense.
Since Sp. Attacks deals damage depending on how hight the users Sp. Attack is and how low the opponents Sp. Defense is.