$'$

The symbol $'$ represents the derivative of a function. If $f(x)$ is a function, then $f'(x)$ is the derivative of that function. It is an expression that models the rate of change of $f(x)$ as $x$ changes. In Leibniz's notation it would be $\frac{df}{dt}$. In Newton's notation, it would be $\dot x$.