A back link is a any link on another website that is not your own that points to a page on your website. This is different from an internal links which is when you link one page on your website to another page on your website. They are helpful in SEO because the search engines, particularly Google, sees links as "votes". Whenever another website links to yours they are essentially "voting" for your web page, or a better way to say it is, they saying they believe your web page is very helpful or informative on the topic it covers.
Backlinks carry weight because they are generally out of control of the receiving website. (As a general rule, the less control a webmaster has over something and the more visible it is to users the more weight it will carry). Yes, you can attempt to make websites just to link back to your own website but there are many problems with this. Not only can you not make enough links to make a difference regardless of any other factors, but the search engines can sniff out networks built for such a purpose and devalue those links completely and/or ban those sites from their search engines.
Additionally, not all links are created equal. Links from related web pages are worth more then from unrelated pages. The anchor text in those links are also a factor in the link's value as is the page's ranking for the search terms you wish to rank well for. In Google PageRank is also a factor (although much smaller then many would have you believe). So, going back to my artificial network example, even if you make a network of sites just to link to yours, because they won't have any backlinks of their own and will rank poorly their links to you will be very low value and not helpful at all. Especially if they are off topic. If you try to make sites similar to your own you have two problems: either the sites contain duplicate content and will be ignored by the search engines or you'll be wasting good content on a satellite website when it could be doing your main website good by attracting links where you want them in the first place.
The best way to get backlinks, especially quality backlinks, is to have high quality content on your website. These naturally attract links as webmasters prefer to link to good content and not frivolous content. Building up quality backlinks takes time and never goes quickly. Anyone telling you otherwise is not well informed or being dishonest.