Hey, how's it going?Good to see you again!

• A friend has come to visit you - you haven't seen him for a long time.
• Hey, how's it going? This is very informal and colloquial. It means hello, how are you?
• Good to see you again. Short for it is good to see you again. It's very common to omit the pronoun / verb in this way - e.g. Nice day today, isn't it? = it's a nice day...
• Again... obviously, you have both met before. If you haven't met before, you can say good to see you or good to meet you.
• Good to see you again is said when we haven't seen someone for a long time.