For something to be a "thing" would be saying that something is now a trend. This is not just limited to fashion but can also refer to mannerisms during social interactions, a blown-up slang or phrase from a movie, etc. For instance, "Hey, apparently wearing khaki shorts is a thing now. Did you know?" is a good example where the phrase could be applied. However, it is important to note that to say that "x is a thing" is an informal manner of speech.

Hard to explain. In this expression it means something like "a big deal", "a situation". In American style dating saying "i love you", is something that means a lot. Usually people are careful about when they say it for the first time. Friends are always asking if someone in the couple said it. and then they will talk a lot about why they did or did not. and try to figure out what the person is thinking based on who said it, when they said it, the situation, their voice. so, it is a very complicated thing.