Let me try to help you out. Im not going to talk about speed, agility, etc. because they are pretty obvious but here is my take on what these categories mean:
Trucking- Ability to fall forward and gain extra yards when tackled.
Release- Ability to get off the line while being pressed.
Impact Blocking- Ability to "Pancake Defenders"
Block Shedding- Ability for a defensive player to disengage from an offensive lineman and make a break for the ball.
Ball Carrier Vision- Ability for a player to see openings when they have the ball.
Pursuit- Ability to take proper angles when trying to tackle the ballcarrier.
Play Recognition- The ability for a defensive player to read the play and sense whether the ball is handed off or it is a play action.
These are what I understand these ratings to mean. Sorry if I wasnt any help! This is my first post on these forums and I gotta say I love this site and I love the amount of "sim" players there are out there. If you have any other questions then let me know and I will try my best to help you out.