It is called the member initialization list, and what it does is initialize your member variables at construction, as opposed to after construction. It is like:
int num = 0; (initializer list)
vs.
int num; num = 0; (assignment after initialization to bogus value)
Initializer lists can be useful for example when you have constant member variables (which cannot be assigned a value after construction).
To elaborate a bit more it's particularly useful when you have objects where the constructor and assignment operators are expensive. If you initialise the object in the constructor then the default constructor is invoked when the container is created, followed by the assignment inside the container's constructor. If you use the initialiser list only the constructor of your choice will be executed.
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