Hello there, please help me with the question.
What does the ＂it＂ mean, when we say " worth it".
For example,

" I'm finally back home now, feeling really tired, but celebrating Naadam with my friend was totally worth it. "

What does the " it " mean,when it is used after the worth?

Question

Hello there, please help me with the question.
What does the ＂it＂ mean, when we say " worth it". 
For example,

"  I'm finally back home now, feeling really tired, but celebrating Naadam with my friend was totally worth it. "

What does the " it " mean,when it is used after the worth?

Answer

I agree with Claudia. FYI, 'Worth it' can also be used, even if the 'IT' is not included in the the sentence.  IT just refers to whatever effort, cost, time, difficulty etc. you had to do in order to do the think being talked about.

Answer

"It" in this specific sentence refers to being tired. Even though you got tired, you had a good time.

In other words, getting tired was worth, since you enjoyed celebrating after all.

Answer

The sentence means the total experience (celebrating) paid back all its costs (the effort that was necessary).

In the sentence "X is worth it", "it" refers to the cost or effort that X requires.  Example:

"Running a mile is worth it" means that the benefits of running a mile justify the effort.

Answer

It refers that “ everything in the world has its price “ :) 
Very tired = exhausted