More formally, we can state the Transfinite Recursion Theorem as follows. Given a class function $G\colon V\to V$, there exists a unique transfinite sequence $F\colon\mathrm{Ord}\to V$ (where $\mathrm{Ord}$ is the class of all ordinals) such that $F(\alpha) = G(F\upharpoonright\alpha)$ for all ordinals $\alpha$. (Wikipedia, transfinite induction)
First question is, what does $\upharpoonright$ mean? Also, what exactly is $F$ in this usage? $F$ seems to be some form of function, but it says its transfinite sequence...