W/T is thread width divided by thread height.. If the term "aspect ratio" is one you know, it's basically the same thing.
if w/t = 1.0, threads are as tall as they are wide
if w/t = 1.5, threads are 50% wider than they are tall (1x tall, 1.5x wide)
if w/t = 2.0, threads are twice as wide as they are tall (1x tall, 2x wide)
In general, W/T less than 1.0 doesn't make sense - it's hard to make the nozzle put down a thread that is thinner (in x/y) than it is tall.
So, anyway, "thinner threads" is a little tricky because w/t is a ratio. A w/t = 1.5 on a 0.3mm layer height makes threads that are 0.45mm wide. A w/t = 1.5 on a .2mm layer height makes threads that are 0.3mm wide.
Usually, I pick what thread width I want then figure out the w/t based on the layer height. If I want threads that are 0.4mm wide, I divide that by the layer height and that tells me w/t. So.. for 0.4mm wide threads with 0.1mm layers, it's w/t = 4.0; for 0.4mm wide threads with 0.2mm layers, it's w/t = 2.0.
I haven't tested how thin you can go with the stock ultimaker 0.4mm nozzle but I suspect (against reason) that it's about 0.3mm. Not sure what the upper limit is but I'd guess around 0.7mm.