The phrase may reference activities or notions specific to a culture, which might not be globally recognized. You haven't heard about Thanksgiving dinner? It's a thing in the US.
Pro Tip 2/3
Subjective Nuance
Used to express personal identification with a practice or trend, which might not be universally understood. I always eat ice cream after a breakup—it's a me thing.
Pro Tip 3/3
Trend Referencing
Can acknowledge new or current trends that might not yet be well-known. Have you heard about tiny houses? It's a thing now.