Can someone explain Microsoft Unity?

Unity is a library like many others that allows you to get an instance of a requested type without having to create it yourself. So given.

public interface ICalculator
{
    void Add(int a, int b);
}

public class Calculator : ICalculator
{
    public void Add(int a, int b)
    {
        return a + b;
    }
}

You would use a library like Unity to register Calculator to be returned when the type ICalculator is requested aka IoC (Inversion of Control) (this example is theoretical, not technically correct).

IoCLlibrary.Register<ICalculator>.Return<Calculator>();

So now when you want an instance of an ICalculator you just...

Calculator calc = IoCLibrary.Resolve<ICalculator>();

IoC libraries can usually be configured to either hold a singleton or create a new instance every time you resolve a type.

Now let's say you have a class that relies on an ICalculator to be present you could have..

public class BankingSystem
{
    public BankingSystem(ICalculator calc)
    {
        _calc = calc;
    }

    private ICalculator _calc;
}

And you can setup the library to inject a object into the constructor when it's created.

So DI or Dependency Injection means to inject any object another might require.