For different types of businesses, buyer needs at the problem/need recognition stage – top of  the funnel (TOFU) – are different. If you’re running a consulting business, for example, then your clients already realize that they’re having certain problems around your service area – like a high cost per lead (if you’re in marketing) or disorganized spending (if you’re in accounting).
If you're wondering what a sales funnel is, simply imagine a real-world funnel. At the top of that funnel, some substance is poured in, which filters down towards one finite destination. In sales, something similar occurs. At the top, lots of visitors arrive who may enter your funnel. However, unlike the real-world funnel, not all who enter the sales funnel will reemerge out from the other end.