Hidden lines show edges of the object that exist behind a surface in the current view. They're drawn as short evenly-spaced dashes (typically 3 to 5 mm dashes with 1 to 2 mm gaps in AS 1100 conventions) and at a thinner weight than visible-edge lines.
For a gate front elevation, hidden lines show the back face of the frame, the back face of any battens, and the post sections behind the gate face. They let the reader understand what's structurally there without cluttering the visible-edge geometry.
Too many hidden lines crowd the drawing and become noise. The convention is to omit hidden lines when they don't add information: if a hidden detail is shown clearly in another view, it's better to leave it out of this one.