50wt is the standard weight thread for piecing. Mettler is a 2-ply thread, meaning it will be even thinner than the more common 3-ply thread (although rumor has it that all manufacturers are heading toward 2-ply thread eventually). It is also used for quilting -- though many quilters prefer 40wt as it will show up more & therefore feature the quilting design better. Thread weights are opposite of human weights. The smaller the number, the fatter the thread. So 100wt is the thinnest & 12wt is the fattest.
Mercerized thread just means that some of the fuzzies have been burned off the thread. It's better for your machine. "Silk finish" is the chemical finish they use to make it a bit shinier. There is no silk in the thread.
http://www.mettlerthreads.co.uk/acat...inish547m.html
Mettler is a good brand for sewing & quilting. Give it a try!