merge option

hey guys.I have a question :“what does merge do in mirror modifier”?
thanks.

Where a vertex is in the same place (within the Merge Limit distance) as its mirror it will be merged with the mirrored vertex.

http://www.blender.org/manual/modifiers/generate/mirror.html

Merge http://www.blender.org/manual/modeling/meshes/editing/basics/deleting.html?highlight=merge

In other words, if multiple vertices are within the merge distance they will be all joined together into a single vertex

Than you for your help Richard.