From BlenderWiki

This sub-panel appears in the Editing Context panel group which is accessed using F9 or clicking button in the Buttons window. This sub-panel is part of the Modifier parent panel. For further information about the common panel components see the Interface section on modifiers.

[edit] Mirror

• Merge Limit: - The distance before welding of vertices begins. The vertices will snap to together when they reach the distance specified.
• X,Y,Z - The mirror axis.
• Do Clipping - Prevents vertices from crossing through the mirror Plane.

Many modeling tasks involve creating objects that are symmetrical. However there's no quick way to model both halves of an object without using one of the workarounds that have been discovered by clever Blender artists for years.

A common technique is to model one half of an object and use Alt D to create a linked duplicate which can then be mirrored on one axis to produce a perfect mirror-image copy, which updates in realtime as you edit.

The Mirror Modifier automatically mirrors your mesh along the X, Y or Z axis which passes through the object center. It then welds vertices together along that axis within a tolerance distance you can set via the Merge Limit button.

If you want to prevent vertices from the original object from moving across or through the mirror Plane then activate the Do Clipping feature.

Once your modeling is completed you can either click Apply to make a real version of your mesh or leave it as is for future editing.