Lean Startup is a collection of practices - mainly for startups - that want to create and roll-out a new product to the market. It puts emphasis on learning about your target customers, their needs and the market. It favors learning over the engineering art or brilliance of the founder's idea. It aims to verify the critical assumptions of a business plan as quickly and as cheaply as possible.
Cheap, Powerful Technology
We now live in a world where technology is cheaper than ever. It's now the easiest time in history to launch an online business. More and more entrepreneurs ("start-upers") are doing exactly that...
People Have Everyghing They Need
At the same time people's needs are satisfied to a level unprecedented throughout human history. From food and housing, through cars and appliances, to telecommunications and information. In the western world most people have everything they need and most of what they want...
This creates an oversupply of new products and services. New businesses need to compete fiercely for their client's attention and the technology is rarely the main obstacle. (in some cases it is, but that's a different story)
Enter the Lean Startup
This is why areas like achieving problem/solution fit and product/market fit are becoming critically important. The sooner you know you're customer's problems, the sooner you confirm target market exists, the less time and money you'll waste chasing the wrong ideas.