A Yuga is like a big cycle of time that goes on and on forever. It's like a really, really long story where things start off great and then get worse and worse before finally getting better again.
There are four different parts to a Yuga: the Satya Yuga, Treta Yuga, Dvapara Yuga, and Kali Yuga. Each Yuga lasts for a really, really long time - like thousands and thousands and thousands of years.
During the Satya Yuga, everything is really good. People are happy, there's no fighting, and everyone loves each other. But then over time things start to get worse.
In the Treta Yuga, people start to get a little more selfish and forget about how important it is to be kind to others. In the Dvapara Yuga, things get even worse and people start to fight and argue with each other.
Finally, in the Kali Yuga, things are pretty bad. People are really selfish and don't care about anyone else. There's a lot of violence and crime and it's hard to feel safe.
But don't worry! After the Kali Yuga, things start to get better again and the Satya Yuga starts all over again!