Session hijacking is like a sneaky thief stealing someone's identity online. The thief tricks the computer into thinking it's the real user and gains access to their accounts or sensitive information. This can happen by spying on internet traffic, injecting harmful code into websites, or pretending to be the user during online sessions. To prevent this, websites need to use strong security measures, like encryption and monitoring for suspicious activity. Users should also be careful about where they browse and keep their software updated to stay safe from these cyber crooks.