Changes the current component selection by selecting the polygon edges associated with the original selection and then extends the selection of the edges as an edge loop in either direction along the polygon mesh.
An edge loop is a path of polygon edges that are connected in sequence by their shared vertices.
For example, if you have a polygon mesh comprised of four-sided polygons (quads) with one interior vertex selected and then choose Select > Convert Selection > To Edge Loop, four edges initially become selected on the mesh (based on their connection to the selected vertex) and then the edge loop selection is extended outward in both directions along the mesh to define a path of selected edges.
Many of the convert selection features are available from the context sensitive polygon marking menu when you press 
+ 
whenever a polygon component is selected.