What is the meaning of this constructor? And why is it a constructor?

Question

public List(int capacity=defaultCapacity) {
items = new T [capacity] ;
}

Answer 1

A constructor was used the perform initial setup or logic need to be performed when the object was created/instantiated and have the same name as its class.

Just like methods, constructor can be overloaded with different parameters like the example you've given — It use an optional parameter to setup the object.

Therefore the following statements will trigger the constructor above:-

var listA = new List();
var listB = new List(123);

P/S: This is an oversimplification as the defualt implementation of List in C# include generics.