这个轴环境的各个部分起什么作用？

Question

\usepackage{tikz} 
\usepackage{pgfplots}
\begin{tikzpicture}
  \begin{axis}%
    [grid=both,
     minor tick num=5,
     grid style={line width=.1pt, draw=gray!10},
     major grid style={line width=.2pt,draw=gray!50},
     axis lines=middle,
     enlargelimits={abs=0.2}
    ]
    \addplot[domain=-1:1,samples=100,smooth,red] {sin(deg(pi*x))};
    \addplot coordinates {
        (0,0)
        (1,1)
        };
    \end{axis}
\end{tikzpicture}