Mirrors sketch geometry across a centerline.

To begin, use commands on the Sketch tab to create the geometry to mirror.
Pattern panel
Mirror
. The sketch geometry is mirrored, using the mirror line as its mirror axis. Symmetric constraints are applied between the mirrored geometry. You can delete or edit segments after you mirror them, and the remaining segments are still symmetrical.

With Self Symmetric selected, the Mirror command creates a single spline that is symmetric about the mirror line. Available only when the geometry you select is a spline, and crosses the mirror line.
Pattern panel
Mirror
.