Hi Amra,
A seed mesh is a starting mesh that you specify so that HFSS will spend less time "guessing" about how to construct the appropriate mesh, and converge more quickly.
You can set this up by right clicking an object and selecting "Assign Mesh Operation" -> "Inside Selection". A good rule of thumb I use is to assign a distance that is half of the minimum dimension of the object. I usually assign these seed meshes to the vacuum regions in between the metal traces.