documentclass[10pt, a4paper]{article}

usepackage{pgfplots}

begin{document}

begin{tikzpicture}
begin{axis}[xmin = -5, xmax = 5,
          ymin = -1.5, ymax = 1.5]

addplot[mark = none] {tanh(x)};

addplot[mark = none, dashed] {1};

addplot[mark = none, dashed] {-1};

end{axis}

end{tikzpicture}

end{document}