จง is mostly used when giving instruction. You will see it a lot in exercise book. For example "จงนำคำต่อไปนี้มาเติมในช่องว่าง" which means "Complete the sentence with these words" 

It's kinda like 'must'(ต้อง) but not the same it's give different feeling.

it's in command type of word

if :: have to(necessary)[ต้อง] -> must(very necessary)[ต้อง] -> very very must do[จง]

 http://i.imgur.com/RWG6siW.png

 See picture for full, I don't know why it not let me submit.