M1 Velocity time graph quadratic

I'm trying to do a variable acceleration question and I'm very confused about velocity time graphs. I understand that when the velocity is negative, the particle has changed direction, but I'm confused about the bottom of the parabola. What does the very lowest velocity value mean? I would have thought velocity would have to equal zero again for another change of direction but that's not until later in my parabola. I'm very confused. Please help!