Usually asked about a particle moving along the x-axis. x(t) is position, v(t) velocity, a(t) is acceleration. Displacement x(b)-x(a), average velocity using average rate of change x(b)-x(a)/b-a. If velocity is negative, we're moving to the left. If velocity is positive, moving to the right.
If acceleration and velocity match sign, speeding up. If acceleration and velocity mismatch, slowing down. Greatest speed is the largest velocity, positive or negative.