Top of the funnel refers to the very first stage of the buying process. Leads at this stage are just identifying a problem that they have and are looking for more information. At this point marketers simply want to get known by the people most likely to be having the kind of problem they have solutions for.
An example of a lead at such a stage is a student whose senior six results has just been released and his/her parent. The lead (student and parent) in this case has realised the need to get university admission (early).