null can have different meanings:
Object variables that are uninitialized and hence point to nothing. In this case, if you access members of such objects, it causes a NullReferenceException.
The developer is using null intentionally to indicate there is no meaningful value available. Note that C# has the concept of nullable datatypes for variables (like database tables can have nullable fields) - you can assign null to them to indicate there is no value stored in it, for example int? a = null; (which is a shortcut for Nullable<int> a = null;) where the question mark indicates it is allowed to store null in variable a. You can check that either with if (a.HasValue) {...} or with if (a==null) {...}. Nullable variables, like a this example, allow to access the value via a.Value explicitly, or just as normal via a.
Note that accessing it via a.Value throws an InvalidOperationException instead of a NullReferenceException if a is null - you should do the check beforehand, i.e. if you have another non-nullable variable int b; then you should do assignments like if (a.HasValue) { b = a.Value; } or shorter if (a != null) { b = a; }.
The rest of this article goes into more detail and shows mistakes that many programmers often make which can lead to a NullReferenceException.
