Distinguish distance from displacement and average speed from average velocity, and interpret slope on a position-time graph.
Kinematics is the description of motion -- not yet worrying about what causes it (that starts with Newton's laws in a few weeks). The starting point is position: where an object is relative to some chosen origin, which can be positive or negative. Displacement is the CHANGE in position (final position minus initial position) -- a vector, since it has a direction built into its sign. Distance is the total length of the path traveled, a scalar that can never decrease. Someone who walks 40 m to the mailbox and back has traveled a distance of 80 m but has a displacement of 0 m, since they ended up right back where they started.
The same split applies to rate of motion: average speed is total distance divided by total time (always positive), while average velocity is displacement divided by time (a vector, which can be zero or negative). On a position-time graph, the slope of the line IS the velocity -- a steep line means fast, a flat line means at rest, and a line sloping downward means moving in the negative direction. This graphical reading skill will resurface all year.
Mastery looks like: They correctly compute distance, displacement, average speed, and average velocity for a multi-part trip and read direction and speed directly off a position-time graph's slope.
Common stumbles: Treating distance and displacement as always equal (true only for one-directional motion); reporting average velocity without a sign or direction.