Class struggle is like when we play games with our friends and some of us have more toys or better toys than others. The ones with more or better toys are like the "rich" ones, and the ones with less or worse toys are like the "poor" ones. Sometimes, the rich friends might not want to share their toys or might even take away toys from the poor friends, and that's not fair.
In real life, there are people who have more money, power, and resources (like toys) than others, and they are usually part of a higher social class. People who have less money, power, and resources are usually part of a lower social class. The struggle comes in when the higher and lower classes don't agree on how things should be done, and they fight for what they think is best for themselves and their group.
For example, the rich might want to keep their money and power to themselves, and the poor might want more help and opportunities to improve their lives. This is a very basic explanation of class struggle, but it's kind of like when we play games with our friends and have to make sure that everyone has a fair chance to enjoy the game and the toys.