Merge Components to Center merges the selected vertices so they become shared, and any associated faces and edges are also merged. The resulting shared vertex is positioned at the center of the original selection.
Note: Merge Components to Center merges to a central point based on the center of the selection. If you want to collapse edges on a component by component basis, select the edges, then select Edit Mesh > Collapse, or choose Merge/Collapse Edges > Collapse Edge from the marking menu. ( 
+ 
-click an edge.)
+ -click an edge.)