The given formula describes a parabolic trajectory \( y(x) \), which the body follows after being thrown / launched. Using this formula you can calculate the current height \(y\) for each current horizontal position \(x\) of the body.
This formula is a special case of the inclined throw. And the minus sign indicates that the body falls downwards (set as negative direction).
It is assumed here that this velocity does not change during the flight, that is, it is constant at all times.