After changing to your mathica directory and opening Mathematica by entering math, In[1]:= will appear on
the command line. In general, In[n]:= indicates the nth input to Mathematica in your current session. You terminate
your input by hitting the ENTER key. Input too long for a
single line can be typed continuously. In some cases, Mathematica can recognize when input lines are not complete, say
when parentheses are missing. You could begin input with a (
to force continuation on input too long for a single line. When the
input is finished, end with a ) and press ENTER. For
each input In[n] line there will appear a corresponding output
line of the form Out[n] = 
which will give the output
produced by the input to In[n]:=.