Okay, so imagine you want to prove something to your friends, like that your toy is the best toy. You have to give reasons to show that your toy is the best. That's kind of like what itmam al-hujjah is.
Itmam al-hujjah means providing enough evidence or arguments to completely prove a point or argument. It's like building a big tower of blocks, and every block is a reason that proves your point. When you have enough blocks, you can say "itmam al-hujjah!" which means "I have completed the proof!"
It's important to do itmam al-hujjah when you want to make sure people believe what you say, and it's especially important when talking about big things like religion or politics. So, just like building a big tower of blocks, you need to be patient, and really think about all the reasons and evidence you can use to prove your point.