Well, when we talk about hacking, we're talking about someone who tries to get into a computer or a website that they aren't supposed to be in. It's kind of like if you tried to sneak into a room in your house that your mom told you not to go into. Only it's way more complicated than that.
There are different kinds of hackers, like the ones who use their skills to help protect computers and websites (we call them "white hat" hackers), and then there are the ones who use their skills to do bad things (we call them "black hat" hackers).
Sometimes, hackers use special programs or code to try to find weaknesses in a computer or website's security. They might use something like a "password cracker" to try to guess a password, or a "DDoS attack" to overwhelm a website with too much traffic so that it shuts down.
Hacking is generally frowned upon because it's illegal and can cause a lot of damage. But the good news is that there are also people out there working hard to make sure that computers and websites are as safe and secure as possible, so that hackers can't get in.