The drag force ($F_d$) on an object in a fluid is given by: $$ \begin{equation}\begin{aligned} F_d=\frac12 \rho v^2 C_d A\\ \end{aligned}\end{equation} $$
Where $\rho$ is the density of the fluid, $v^2$ is the flow velocity relative to the object, $A$ is the reference area and $C_d$ is the drag coefficient.
A simpler format for this is: $$ \begin{equation}\begin{aligned} F_d=kv^2\\ \end{aligned}\end{equation} $$
Where $k$ is a placeholder for the other factors.
Notice that the drag force is directly proportional to the square of the velocity of the object ($F_d\propto v^2$). This means that by doubling the velocity, we effectively quadruple the drag force on the object.
Because of the increasing drag force on an object moving through a fluid, there will be a state in which the velocity ($v$) and thus the drag force ($F_d$) increases to the point where the drag force cancels out the force ($F$) that is causing the object to accelerate. The object thus reaches a terminal velocity.