So what are back-end products? They are products/services you offer to existing customers, i.e. to people who have already bought a first product (front-end product. lead product) from you.
Most online marketers make much more money selling customers the second, third, fourth etc. product than selling their first product. The reason is that people who've already bought from you once are much more likely to buy again.
Strategies like back-end offers shifts the focus from the short sighted "take the money and run" strategy that is so much in use on the internet today. You've seen these sites all over. The focus is on getting a massive amount of traffic and then selling visitors an over-priced product that doesn't deliver what was promised. They might make money in the short run but they will only sell to each customer once and will have to continue spending a lot of money on advertising to get new suckers to visit their site.
More successful sites focus on building a strong relationship with their customers. Your main goal shouldn't be to just make sure your customers are somewhat satisfied...you want them to be extremely satisfied. If you deliver the goods, your customers will trust you more. If you have their trust, you can sell them anything. When you send your very satisfied customers an email offering another product that they would be interested in, they will flock to your site to buy it because they trust you. Trust is everything.