Vector

Two pieces of information, always: how much, and which way. Position, displacement, velocity, acceleration, force and momentum are the vectors an AP course leans on, which EK 1.1.A.3 lists against the scalars.

Notation. An arrow over the symbol marks the vector, $\vec{v}$, and the bare letter $v$ means its magnitude. EK 1.1.A.3.ii adds the exception that matters most in practice: vector notation is not required for a component along an axis, because in one dimension the sign of the component completely describes its direction. So $v_x = -4$ m/s is already a complete vector statement, provided you have said which way is positive.

Declare the positive direction before writing a single sign. A vector sum in one dimension is ordinary addition with opposite directions given opposite signs (EK 1.1.B.1), and an axis that flips halfway through a solution is the most expensive error in mechanics.

Drawn, a vector is an arrow whose length is proportional to its magnitude and whose direction is the quantity's direction (EK 1.1.A.2). A magnitude is never negative: an acceleration component of $-9.8$ m/s$^2$ has magnitude 9.8 m/s$^2$.

In two dimensions you stop adding numbers and start adding components. Topic 1.1 Scalars and Vectors in One Dimension covers the one-dimensional case.