(Algebraic Geometry and commutative algebra - Bosch - page 16)
I could not find the symbol in the glossary at the end of the book. What does it mean ?
I cannot see it in my reference manual either (Commutative Algebra with a view towards algebraic geometry - Eisenbud)
Its the disjoint union from set theory. Each element of $\text{Spec } R$ maps into a different set from the disjoint union.
In category theory this symbol can sometimes be a coproduct, but in this situation it’s not.
Added: Of course disjoint union is the coprduct in sets, but you hardly see all those rings/ $R$-modules and think “category of sets”. This tradition of confusing young scholars of scheme theory goes back, at least to Hartshorne’s book.