The 'what' before the 'why'
A ball rolls across the floor, a car pulls away from a light, rain streaks down a window. Every one of these is motion, and physics splits the study of motion into two jobs. The first job is simply to describe it accurately — where is the object, how fast is it going, in what direction, is it speeding up? That job is kinematics, and it is where we start.
You can only describe motion relative to something
To say where something is, you need a reference: an origin to measure from and axes to point along. That choice is a frame of reference. A passenger sitting on a moving train is at rest relative to the train but moving at 100 km/h relative to the ground. Neither statement is wrong — motion is always described relative to a chosen frame, and choosing a convenient frame is half the art of solving a problem.
Once a frame is fixed, an object's position is its location relative to the origin. In one dimension that is just a single number with a sign — say x = +4\text{ m} to the right of the origin, or x = -3\text{ m} to the left. Position is our first vector quantity: it carries a direction, not just a size.
Distance and displacement are not the same thing
This is the first place beginners slip. Distance is the total length of the path you actually travelled — a scalar that is never negative. Displacement is the straight-line change from start to finish, with direction — a vector. Walk 3 m east, then 4 m back west: your distance is 7 m, but your displacement is only 1 m west.
Displacement is the final position minus the initial position. The Greek capital delta, Δ, always means 'change in'.
Motion diagrams: a strobe photo of movement
Physicists have a wonderfully simple picture that captures a whole motion at a glance: the motion diagram. Imagine photographing a moving object with a strobe light that flashes at equal time intervals, so each flash marks the object's position. The pattern of dots tells you everything. Even spacing means steady motion; spacing that widens means speeding up; spacing that closes in means slowing down.
The road ahead
From here the track climbs in order: velocity (how the position changes, and the slope of a graph), then acceleration and the equations of motion (the workhorse formulas of first-year physics), then projectile motion in two dimensions, and finally relative velocity and full worked problems. Master this description of motion and the rest of mechanics — forces, energy, momentum — has a solid floor to stand on.