Best explained with an example. I wanted to replace my gas stove [made of steel], but was unaware that glass-top type gas stoves are "in" these days. How does one ascertain this claim made by a shopkeeper or a friend ["aaj kal kya/ye chal raha hai"]? Do a quick search on Flipkart, Snapdeal or Amazon or others and just glance at the top few results. That's the stuff which is "in" and "trending" these days.