So, imagine you and your friends are trying to decide what game to play. You all want to play different games, but there's one friend who keeps telling you all to play the same game. You later find out that this friend was actually being told what to say by another person who doesn't even go to your school.
In a similar way, there are people who think that the vote in 2016 that decided if the UK should leave the European Union (Brexit) wasn't fair because someone from another country, Russia, might have been telling people what to do or saying things to make them change their minds. Basically, they think that Russia might have meddled in the decision-making process, like that friend who was being told what to say about the game.