Palentologists and the like, what do you mean by the missing link?

in human history i have often heard about the mystery of the missing link and how it has never been found with certain undeniable facts. ok a summary of this point would be great. but what exactly is the period that causes the so called missing link in the annals of human history.to go missing. what did we have knowledge of some sort of ape, then onto homosapians, or whatever. and how old is the human race. if we derived from the sea do we go back that far in determining the age of humans. or do we mean humans in the form we all recognise. thanks for all your help on this question. much appreciated.