The Brief: Big mood means "relatable."

DEEP DIVE
The phrase can be used on its own as a caption to a photo or video, or as a part of a sentence, as in saying that something “is big mood.” It is synonyms with and possibly an evolution of “mood.”
Big mood is also in the same vein as TFW, goals, and me RN.
big mood pic.twitter.com/Hq01JG4dnv
— Phos Marked For Death (@Phosphatide) January 29, 2019
Dua lipa wearing a massive bubble coat is a big mood today pic.twitter.com/Ae8Sf2R14M
— Jack mull (@J4CKMULL) January 30, 2019