A backlink is just one of the measures used by Google and other search engines to determine where your hub or other webpage should appear in the rankings.
A simple way of viewing it is to think of it rather like a 'vote' for your page or site and the higher the page rank of the page your 'vote' comes from, the more value the search engines attribute to that link. So, not all backlinks are created equal and some are worth a lot more than others!
A link can be made in many ways, other webmasters could decide that your web page has valuable information that they want to pass on to their readers. Someone surfing the net could decide to bookmark your page on their facebook account or 'tweet' it. Or you could create your own backlinks by bookmarking your pages, leaving comments on other articles, pages or hubs or by article marketing - there are other ways but this is supposed to be a brief answer!
A backlink is a clickable link somewhere on the web that when clicked will lead the person to the page you are backlinking to. This can be a clickable URL (or webpage address - what appears in the browser window when you are on a webpage) or it could be anchor text where you use keywords or keyword phrases that you want to get your page ranked for and you make these keywords what is referred to as Anchor Text - either by using the link symbol (as when editing your hubpage) or by inserting a simple bit of html code your keywords then what will show is just your keywords and these will become the clickable link.
I have written some hubs on backlinks and creating anchor text links if you would like to take a look.
Yahoo site explorere was formerly a great place to check your backlinks however this is being scaled down and phased out. I am now using a free tool called Open Site Explorer which you can find here http://www.opensiteexplorer.org and I hope you find this useful as it also gives some other helpful measures about your site's authority in the eyes of the search engines. You can use it for individual hubs or webpages as well as the root domain name of a site (the .com or .co.uk address).
You should bear in mind though that backlinks take a while to show up in a tool like this so don't get disheartened and think your backlinking efforts aren't working if you do not immediately see results in Open Site Explorer.