Well, to understand semi-arianism, we first need to talk a little bit about Christianity. Do you know who Jesus is? He is an important figure in Christianity and is believed by Christians to be the Son of God.
So now, some Christians believe that Jesus is fully God, while others believe that he is only a human being. We call these two groups of Christians "Trinitarians" and "Unitarians".
Semi-arians, on the other hand, believe that Jesus is not fully God, but he is also not just a human being. They believe that Jesus is divine, but not equal to God the Father.
It's a little complicated, but think of it like this: Imagine you have a toy car that looks just like a real car. The toy car is still a car, but it's not exactly the same as a real car. It's similar, but not the same. Semi-arians believe that Jesus is like the toy car - he is divine like God, but not exactly the same as God.
Does that make sense? Let me know if you have any more questions!