According to the Wikipedia article on Riemann hypothesis,
The number of zeros of the zeta function with imaginary part between $0$ and $T$ is given by $$N(T)=\frac{1}{\pi}\operatorname{Arg}\left(\xi\left(\frac{1}{2}+iT\right)\right)$$ where $\xi$ is Riemann's xi function and the argument is defined by varying it continuously along the line with $\Im (s)=T$, starting with argument $0$ at $\infty+iT$.
What exactly does "varying it continuously along, [...] starting with [...]" mean? I have no idea but I know the definition of the prinicpal complex argument, how are these arguments related?