If the first thing users need to understand when they arrive is what you do, then the second is how you are different to everybody else. What you do can be shown through imagery and design choices that suggest a target audience. How you are different usually means stating it very clearly.
Explaining quickly how you are different is very important because otherwise what reason do people have to stay on your site? Why not just go to Amazon, eBay, Booking.com or another big aggregator where they've already got an account? Your difference should be something that marks you out as special and shouldn’t be something that others can easily offer. Priding yourself on free shipping and free returns isn’t special but now increasingly common. You could try and be the cheapest but this is a hard one to maintain.
Your difference should be a unique selling point and ideally focused on just one thing. Don't muddy the waters and present three or four things. Once you've got it worked out, you should be able to say this thing in only a few words and you should tell users straight away (probably above the fold). Good examples can be niches ("we're the only people specialising in this thing") or special services ("free personalisation of products") or in how the product/service is produced ("no sweats shops, eco-friendly etc").
Users should immediately know why you are worth spending time with. Be careful not to get too clever in your wording so you leave people guessing: inventing words, using hashtags or jargon, and leaving things ambiguous can all lead to this kind of confusion.