how rdf document called well-fromed?

hai...
I'm yanes, and I'm 22 years old.
and I'm a student from Telkom Institute of Technology from Indonesia,
majoring informatics engineering.

now I'm doing my thesis about Resource Description Framework, and I need to
make a RDF document.
when I'm using RDF validator, it is very helpful, but how your RDF
validator validate a RDF document?
and can you tell me how and why a RDF document called well-formed?

it will be very helpful for me if you willing to answer my question

thank you
-yanes-