Websites are documents and applications that live on the internet. They're made out of markup (which tells your computer what to show you), images, and sometimes code (which tells your computer what to do besides show you the markup). You're looking at a website right now! They're very easy to make.
You can look at a website on a phone or a tablet just as easily as you can on a computer, but it might look different or be more difficult to use.
There is a very wide variety of websites. Some are big, actively maintained, and widely known, like Google's website Google, and Facebook's website Facebook. Others are not so well known. Some contain good information, like Wikipedia, and some contain dubious or useless information, like Is X Dangerous. Lots of websites exist to deliver and sometimes sell software. Probably the vast majority are in some way related to a business, although a good portion of the rest are for personal usage, like publishing a journal or sharing pictures of guitars.
Most of the people who make websites track some amount of information about who visits them, how long they stay, and where they come from. A small portion of websites track a huge amount of other information, such as everything else their users do on the internet, but these websites are also the biggest ones and have the most users. So, visiting a website might result in a company or government finding out about everything you look at online.
If you make a website, you can choose if you want yours to simply display text and images, or if you want it to be "interactive." There are some very basic ways to make the website "interactive" included in normal markup, like buttons, but almost anything more complicated than a simple link (i.e. sending the user to another website or another part of your website) means you'll need to include some code—and that code is very likely to be Javascript. Javascript runs inside the part of a computer that displays the website (called a browser), while most website code has to be run on other computers (called servers). The people who write Javascript tend to develop very strong opinions about it.
Aside from Javascript, mass surveillance, and wildly variant quality of information, websites' main danger is that they expose you to the actions and ideas of other people using the websites. A good example of this danger is Facebook, whose website regularly shows users politically-motivated misinformation put there by propagandists from foreign governments pretending to be members of the users' community. Another is reddit, whose actual users regularly express terrible opinions. A third is Twitter, which has the opinion problem but compounded with terrible jokes.
Most websites are not out to harm you, but some definitely are. Instead, most are unnecessary and dumb. There is the Javascript thing, though, so we consider websites to be a little dangerous.
