Well, you pretty much need to have an idea of how html works if you want to build a website. Thats of course... unless you use one of those site builder tools then you don't necessarily need to know any html.
Learning html is very easy and the basics can be learn quite fast. A useful resource which comes to my head straight away is the w3 schools site. It pretty much covers most if not all that you would need to build a site.
Link:
http://www.w3schools.com/html/default.asp