my question is related to this exposition about Shtukas.
In the subsection on Algebraic Geometry, D.Goss is talking about "rational functions without poles on $\mathbb{P}^1$".
I have the following questions:
$\bullet$ What does $\Gamma$ denote? Is this just the restriction of the structure sheaf $\mathcal{O}_{\mathbb{P}^1}$ to the open affine subspace $U$?
$\bullet$ What does he mean by a rational function without poles? How is a pole defined in this context in the first place?
Thank You for Your help,
SDIGR