Here are all access modifiers in venn diagrams, from more limiting to more accessible:

Access Modifier	Diagram
private
internal
protected
protected internal
public

Below you could find more information.

The public keyword makes a class (including nested classes), property, method or field available to every consumer:

public class Foo()
{
    public string SomeProperty { get; set; }

    public class Baz
    {
        public int Value { get; set; }
    }
}

public class Bar()
{
    public Bar()
    {
        var myInstance = new Foo();
        var someValue = foo.SomeProperty;
        var myNestedInstance = new Foo.Baz();
        var otherValue = myNestedInstance.Value;
    }        
}

The private keyword marks properties, methods, fields and nested classes for use inside the class only:

public class Foo()
{
    private string someProperty { get; set; }

    private class Baz
    {
        public string Value { get; set; }
    }

    public void Do()
    {
        var baz = new Baz { Value = 42 };
    }
}

public class Bar()
{
    public Bar()
    {
        var myInstance = new Foo();

        // Compile Error - not accessible due to private modifier
        var someValue = foo.someProperty;
        // Compile Error - not accessible due to private modifier
        var baz = new Foo.Baz();
    }
}

The internal keyword makes a class (including nested classes), property, method or field available to every consumer in the same assembly:

internal class Foo
{
    internal string SomeProperty {get; set;}
}

internal class Bar
{
    var myInstance = new Foo();
    internal string SomeField = foo.SomeProperty;

    internal class Baz
    {
        private string blah;
        public int N { get; set; }
    }
}

This can be broken to allow a testing assembly to access the code via adding code to AssemblyInfo.cs file:

using System.Runtime.CompilerServices;

[assembly:InternalsVisibleTo("MyTests")]

The protected keyword marks field, methods properties and nested classes for use inside the same class and derived classes only:

public class Foo()
{
    protected void SomeFooMethod()
    {
        //do something
    }

    protected class Thing
    {
        private string blah;
        public int N { get; set; }
    }
}

public class Bar() : Foo
{
    private void someBarMethod()
    {
        SomeFooMethod(); // inside derived class
        var thing = new Thing(); // can use nested class
    }
}

public class Baz()
{
    private void someBazMethod()
    {
        var foo = new Foo();
        foo.SomeFooMethod(); //not accessible due to protected modifier
    }
}

The protected internal keyword marks field, methods, properties and nested classes for use inside the same assembly or derived classes in another assembly:

Assembly 1

public class Foo
{
    public string MyPublicProperty { get; set; }
    protected internal string MyProtectedInternalProperty  { get; set; }

    protected internal class MyProtectedInternalNestedClass
    {
        private string blah;
        public int N { get; set; }
    }
}

public class Bar
{
    void MyMethod1()
    {
        Foo foo = new Foo();
        var myPublicProperty = foo.MyPublicProperty;
        var myProtectedInternalProperty = foo.MyProtectedInternalProperty;
        var myProtectedInternalNestedInstance =
            new Foo.MyProtectedInternalNestedClass();
    }
}

Assembly 2

public class Baz : Foo
{
    void MyMethod1()
    {
        var myPublicProperty = MyPublicProperty;
        var myProtectedInternalProperty = MyProtectedInternalProperty;
        var thing = new MyProtectedInternalNestedClass();
    }

    void MyMethod2()
    {
        Foo foo = new Foo();
        var myPublicProperty = foo.MyPublicProperty;

        // Compile Error
        var myProtectedInternalProperty = foo.MyProtectedInternalProperty;
        // Compile Error
        var myProtectedInternalNestedInstance =
            new Foo.MyProtectedInternalNestedClass();
    }

}

public class Qux
{
    void MyMethod1()
    {
        Baz baz = new Baz();
        var myPublicProperty = baz.MyPublicProperty;

        // Compile Error
        var myProtectedInternalProperty = baz.MyProtectedInternalProperty;
        // Compile Error
        var myProtectedInternalNestedInstance =
            new Baz.MyProtectedInternalNestedClass();
    }

    void MyMethod2()
    {
        Foo foo = new Foo();
        var myPublicProperty = foo.MyPublicProperty;

        //Compile Error
        var myProtectedInternalProperty = foo.MyProtectedInternalProperty;
        // Compile Error
        var myProtectedInternalNestedInstance =
            new Foo.MyProtectedInternalNestedClass();
    }
}