In[n] is a global object that is assigned to have a delayed value of the n input line.
Typing In[n] causes the n input line to be re-evaluated.
In[ ] gives the last input line.
In[-k] gives the input k lines back.
See The Mathematica Book: Section 1.2.1, Section 1.3.3 and Section 2.13.1.
See also: InString, Out, $Line, $HistoryLength.