The sum of the vectors is the resultant vector, which is where the net force is directed.
An example would be if you had a ball rolling on a table and you bumped the ball perpendicular to its initial velocity, then the ball would move at a diagonal angle rather than move straight in the direction where you bumped it.
Acceleration is the change in velocity over time, so the acceleration vector tells us how the velocity's direction is changing.
You can use the vertical line test (the blue lines on the drawing) and if the other end or if any part of the line touches the blue line, then it isn’t a function.
