Okay, so imagine there's a big house, and it has a fence all around it. Now, let's say you want to go into that house, but you don't ask the owner's permission. That's called illegal entry.
It's like if you were at a playground, but someone put up a fence around it, and you climbed over the fence to get in without asking if it's okay. That's not allowed.
So, in the real world, countries have borders that are like fences around them. If you want to go into that country, you have to show some papers or ask permission first. If you don't do that and go in anyways, that's illegal entry.
It's important to ask permission because it's a way to keep everyone safe. Just like how you wouldn't want anyone coming into your house without asking, countries want to make sure everyone who comes in will follow the rules and not cause problems.