Alright kiddo, so you know how sometimes adults have to go to work even if they don't want to? Sometimes they even have to go to work somewhere far away from home.
Well, imagine if lots and lots of adults all had to go to work somewhere far away at the same time. That's kind of what happens when a country goes to war.
To make sure there are enough people to do all the important work that needs to be done during a war, many countries have something called a wartime reserve mode. This is when people who are already retired or not in the military anymore get asked to come back and help out.
It's kind of like calling your grandpa out of retirement to come help you with a project because he has a lot of experience and knowledge. Except in this case, it's for really important things like protecting the country and making sure everyone has enough food and supplies.
So during wartime reserve mode, retired or former soldiers can be asked to come back and help with things like training new soldiers, or even serving in combat themselves. Other people with useful skills might be asked to help out too, like doctors or engineers.
Basically, wartime reserve mode is a way to make sure the country has enough people with the skills and knowledge needed to win a war, even if some of them had already retired or left the military.