The Brief: To take an L means to take a loss or fail at something.

DEEP DIVE
“Taking an L” is the abbreviated way of saying that someone “took a loss” or failed at something. The opposite of an L is a W.
You ever walk out of an exam like just astonished with yourself, like “Wow. I really took a hard L on that.”
— Aleman (@theAlemanC) March 27, 2019
It is most often used in reference to school, work, or sporting activities.
Y’all Bron stans are different 🤣
When Bron takes an L:
“but his stats”Jordan takes an L:
“Who took that L”Now I’ll ask you…
Who was it that took 6 different L’s in the Finals?
Who was it that choked in the Finals?
Who is about to miss the Playoffs for the 3rd time? https://t.co/n8tqM7QBhq
— 🏀 The Ballfather 🏀 (@StevenKelsey13) March 19, 2019