Hyper-Text Mark-up Language (HTML) is the 'code' which creates every web page. (To see what it looks like, go to the top of your screen now, find 'View' and click on 'Source'. This will show you the HTML which makes up this AB web page).
If you've already designed a web site then it will be made up of similar code. Since you don't recognise HTML, I assume that you used a program like FrontPage or Web Plus (or one of many other programs which do the same job). What these programs do is to create the HTML for you. (i.e. your own web site will be made up of HTML code, even if you've never seen it).
What 1and1 are telling you to do is to open the home page of your web site in the program which you used to create it. Then go to the place where you want your counter to appear and simply paste the relevant code into the page. (Depending upon the program you're using, you might have to select something like 'Paste Special' rather than just 'Paste'). Then save your page and upload it to your webspace.
(If your 1and1 site allows on-line editing, you'll need to log into your site and paste the HTML code there).
Chris