Before Christ is a time when people used a different calendar than what we use now. The calendar we use now is based on the birth of Jesus Christ, who is an important religious figure. But before Christ, people used a different way of counting the years. For example, the ancient Greeks used a calendar that started in 776 BC, which is almost 300 years before Jesus was born. The ancient Egyptians used a calendar that started around 4241 BC.
So, to understand "before Christ," you need to know who Jesus was. Jesus is someone who many people believe is the son of God. He was born a long time ago in a place called Bethlehem. Jesus taught people about love and forgiveness, and many people think he did miracles like making sick people better and turning water into wine. But eventually, some people didn't like what Jesus was teaching and they arrested him. They put him on trial and sentenced him to death by crucifixion, which means he was nailed to a cross.
After Jesus died, his followers started a religion based on his teachings. They called themselves Christians, which means followers of Christ. And because Jesus was such an important figure in their religion, they decided to start counting the years based on when he was born. So, the year 1 on the Christian calendar is the year of Jesus' birth. Anything that happened before that year is "before Christ."