"Joy Bangla" is a phrase that was used during the liberation war of Bangladesh. It means "Victory to Bengal" in English.
So, imagine you're playing a game with your friends and you want your team to win. You might shout "Go, team go!" or "Let's go, [team name]!" to show your support and encouragement. That's a little bit like what "Joy Bangla" means - it's a way to show support and encouragement for Bangladesh and the people who fought for their freedom.
During the liberation war, people would shout "Joy Bangla" to show their solidarity and to give each other hope. It became a way to say "We're in this together, and we will not give up until we win."
Today, "Joy Bangla" is still sometimes used as a way to express pride and patriotism for Bangladesh, and to remember the struggle of those who fought for the country's independence.