Kinematics describes motion without asking what causes it: where an object is, how fast it moves, and how that changes. Everything later in the course (forces, energy, momentum) is built on reading and writing motion in words, graphs, equations and diagrams, and on splitting 2-D motion into two 1-D problems.
Scalars have only a magnitude (distance, speed, time). Vectors have a magnitude and a direction (position, displacement, velocity, acceleration). In one dimension the sign of a vector component is its direction, so adding vectors means adding signed numbers.
Displacement is the change in position. Average velocity is displacement divided by time; average acceleration is change in velocity divided by time. An object accelerates whenever its speed or its direction changes, and averages over a tiny interval are the instantaneous values.
Motion can be shown in diagrams, graphs, equations and words. With constant acceleration, three kinematic equations apply, and near Earth g = 10 m/s² downward. On graphs: slope of x–t is velocity, slope of v–t is acceleration, area under v–t is displacement, and area under a–t is change in velocity.
What you measure depends on your reference frame. To convert, add the object's velocity relative to a moving frame to that frame's velocity (as vectors). Every inertial (non-accelerating) frame measures the same acceleration.
Any vector can be split into perpendicular components with sine and cosine. Two-dimensional motion is two one-dimensional problems sharing the same time. A projectile has no horizontal acceleration and a constant 10 m/s² downward acceleration.