Viruses are small files that can do nasty things to your computer or laptop. They come in different shapes and sizes: sometimes they encrypt all your files so you can't use your computer anymore, they watch what you are doing on your computer, or they collect essential data and documents about you. Of course, you don't want that, so that's why virus scanners are installed on almost every computer. They can check whether the file you received via email or in a download is known as a virus or looks like other viruses. You also have to deal with viruses as a hacker: sometimes, you have to make them yourself to demonstrate that you can digitally hack into someone's computer. To ensure that the virus is not automatically removed when the victim tries to open it, hackers first check whether virus scanners can see that it is a virus and circumvent that. The website VirusTotal is an example of a website with virus scanners. You can upload the possible virus here, and then all virus scanners will check at the same time to see whether it is a virus or not. So you can upload a file but also view what the website scanned before.