a vector quantity
A vector quantity carries both a size and a direction; you cannot describe it fully without saying which way it points. Velocity, force, acceleration, momentum and displacement are all vectors. 'Drive 5 km' is incomplete; 'drive 5 km northeast' is a vector. An arrow is the natural picture: its length is the size (the magnitude) and where it points is the direction.
Because direction matters, vectors do not add like plain numbers. To add two vectors you place them head to tail and draw the arrow from the first tail to the last head, the 'triangle' or 'parallelogram' rule. Two forces of 3 N and 4 N give a total of 7 N only if they point the same way; at right angles they combine to 5 N (from 3^2 + 4^2 = 5^2); pointing opposite, just 1 N. The magnitude of a vector v is written |v| and its direction is often given as an angle.
The whole language of mechanics is built on vectors, which is why we split them into components (along x and y axes) to do the arithmetic cleanly. Forgetting that a quantity is a vector, and adding magnitudes as if directions did not exist, is a classic and costly mistake.
Two 10 N forces on a box: if both push right, the net force is 20 N right; if they push at 90 degrees to each other, it is about 14 N (10 sqrt 2) diagonally; if they oppose, it is 0. Same sizes, very different results, because of direction.
Vectors add by geometry, not by summing their sizes.
A vector's magnitude is always zero or positive; the direction carries the 'sign'. Reversing a vector's direction is what a minus sign means, as in -v.