What is the use of static keyword assigned to a variable that is declared outside of all functions?

They Asked everything in C++, but mainly Focused on Virtual Functions, Virtual constructors, Destructors ,Operator overloading, diffrence b/w deep copy and shallow copy give an example,Templates ,smart pointers, STL.. They asked me 3 coding questions and a birthday finding puzzle as well. delete every alternate node in a linked list. questions related to Game Theory, Game Engines, Design Patterns. Asked to draw class diagram for some design patterns. how vectors, maps, unordered_maps are implemented internally.