Why does the instructor change the name of the field Location to _location when the field changes to private?

This is done purely because of naming conflicts with the private field and the constructor taking in a value in the parameter also named location, when instantiating the field. For example a person class that has a name field:

class Person { private readonly string name; public Person(string name) { name = name } } Here we have an issue with both things being called name. To solve this, we can use

this.name = name

where this is referring to the current objects field of name

or we can change

private readonly string name

to

private readonly string _name;

Exactly as Wayne said, there are some so called "naming conventions".

That means, that some particular accessable variable should be written in a special kind.

Short example:

class Invader
{
    // "private" variables are not accessable from outside the class and are shown by naming conventions with an "_" underscore infront of the lower-case first letter
    private MapLocation _location; 

    // "public" variables are always written with an upper-case first letter.
    public MapLocation Location     
    {
        get
        {
            return _location;
        }
        set
        {
            _location = value;
        }
    }
}