C++ - What is object slicing?

#include <iostream>
#include<string>
using namespace std;

class Base
{
public:
    virtual string print() const
    {
        return "This is Base class";
    }
};

class Derived : public Base
{
public:
    virtual string print() const
    {
        return "This is Derived class";
    }
};

void describe(Base p)  <--- Object Slicing Here.
{
    cout << p.print() << endl;
}

int main()
{
    Base b;
    Derived d;
    describe(b);
    describe(d);
    return 0;
}

Output

This is Base class
This is Base class