Ok kiddo, you know how we have the House of Lords and the House of Commons in the United Kingdom? The House of Lords is where people called peers or lords go to help the government make laws and important decisions.
Sometimes, a peer might not want to be in the House of Lords anymore, maybe because they got too old or they want to do something else. When that happens, we need to find a new person to take their place.
That's called a by-election. It's like a special election where people who live in the same area as the former peer get to vote for a new peer. The new peer can then go to the House of Lords and help make laws and decisions, just like the other peers.
So basically, a by-election to the House of Lords is when we need to find a new peer to take the place of someone who can't be a peer anymore, and the people who live nearby get to vote for who they want to be the new peer.