Okay kiddo, so in Matthew 5:34, Jesus is talking to some people and He says “But I say to you, do not swear at all: neither by heaven, for it is God’s throne.”
Now, when people say swear, they usually mean saying something like “I swear on my mother’s life” or “I swear on my favorite toy” to show that they are telling the truth.
But what Jesus is saying here is that we shouldn’t make those kinds of promises at all. Instead, we should just try to always tell the truth, without needing to promise anything.
And when Jesus says “neither by heaven,” He means that we shouldn’t even make promises by saying things like “I swear to God” or “I swear on a Bible” because those things are important and we shouldn’t use them lightly.
So basically, Jesus wants us to always tell the truth and not make promises we might not be able to keep.