This is a term that sounds scary, and it is, as it can happen when visiting a website that would normally be trustworthy. If you visit a website that has been hacked, it may appear to be acting normal, but it could be secretly scanning your computer or mobile device for known vulnerabilities, installing malicious software through whatever vector it finds, then being able to do pretty much anything. That includes stealing your data, monitoring your behavior, secretly viewing you via your webcam, and more.
If it turns out your device hasn’t been updated or patched recently, malicious software can get installed through a drive-by download attack, allowing a hacker to completely take control of your system. Keep your computer and mobile devices updated, and use current malware/virus protection to stay safe.