C++ :: Vector Syntax - Resize And Reserve

We can construct vector in following ways

vector<int>v1; //create an empty vector
vector<int>v2(arr,arr+4); ///to create vector from an array
vector<int<v3(v2); //throuh copy constructor

But I have come across some program in a forum,they have create vector like this

vector<int>v(10,5)

I am clueless ,is this allowed?,what syntax allows this?

if a vector is create like this

vector<int>v(3);
v.push_back(1);
v.push_back(2);
v.push_back(3)
v.resize(2);

Now I am curious to know what are the changes those will happen in vector v because of resizing it?