What does a dot before the variable name in struct mean?

This is a Designated Initializer, which is syntax added for C99.
Relevant excerpt:

In a structure initializer, specify the name of a field to initialize
with ‘.fieldname =’ before the element value. For example, given the
following structure,

struct point { int x, y; }; 

the following initialization

struct point p = { .y = yvalue, .x = xvalue }; 

is equivalent to

struct point p = { xvalue, yvalue };