Okay, so sometimes countries don't get along very well and they try to harm each other. It's not nice, but it happens. One way they might do this is through something called cyberwarfare. This means they use computers and the internet to attack each other.
Russia is a country that has been in the news a lot lately for doing this kind of thing. Some people say they try to interfere with other countries' elections by hacking into their computer systems and pretending to be someone they're not. They might even spread lies or try to make one candidate look bad so people won't vote for them.
It's kind of like when someone tries to cheat in hide-and-seek by lying about where they are, except it's on a much bigger scale and can have serious consequences for everyone involved.
So, Cyberwarfare by Russia means they are using computers and the internet to try and harm other countries. It's not a nice thing to do, and it can cause a lot of problems.