I'm going through "Quantum Computation & Quantum Information" by Michael A. Nielsen and Isaac L. Chuang, and as a high school student with no previous knowledge, I cannot understand some things about linear transformations on the bloch sphere.
The nice thing about the Bloch sphere is that you can visualize the various rotations without having to think too much about complex numbers.
The $Y$ gate causes a 180-degree rotation around the $y$-axis of the sphere. (Imagine spinning the sphere a half-rotation around the $y$-axis.) For example, if your state is at either end of the $x$-axis, then rotating 180 degrees around the $y$-axis will move your state to the opposite end of the $x$-axis. Same if your state is at either end of the $z$-axis. But if your state is on the $y$-axis itself, then applying a $Y$ gate does not rotate the state — it remains in the same place.
Mathematically, yes, this is represented with complex numbers. If it helps, you can think of the $x$-$y$ plane of the Bloch sphere as sort of like polar coordinates representing the relative phase between the $\left|0\right>$ and $\left|1\right>$ parts of your superposition — the $x$-axis represents the real part, and the $y$-axis represents the imaginary part.