What is a lineage?
A lineage is simply descent from a common ancestor. You can think of it the same way as you are descended from your parents, who are descended from their parents (your great-grandparents).
In evolutionary biology, a lineage is how an organism descends from other organisms over a much larger scale of time.
For example, modern humans are descended from an ape-like common ancestor that we share with chimpanzees. If we go back further in time, that ape-like ancestor shares a common ancestor with a monkey-like ancestor. (Note that I did not say humans descended from chimpanzees or apes: rather both humans and chimpanzees descended from an ancestor that was ape-like and now extinct. This is an important distinction).