Tessellation Multiplier controls the amount tessellation along the surface, allowing more detail to be added where needed. As with World Displacement, for this to be enabled, the Tessellation property must be set to something other than None.
When using World Displacement to expand your object beyond its original bounds, keep in mind that the renderer still uses those original bounds. This means that you may see culling and shadowing errors. To address these issues, you can edit the mesh's Scale Bounds property, although your edits could result in a performance drain, potentially leading to other errors (such as improper shadowing).