L means loss meaning you took the loss/said something dumb. Ratio means the ratio of replies to likes is high meaning people also think you've said something dumb and are arguing with you about it. Comes from Twitter but used more generally now (mostly ironically).
It isn't hard to google something with a plus sign and getting a good answer... just google it. :)
Shown answer from dictionary.com: "What is L + ratio? L + ratio is used as a mocking insult on social media, often in reply to a post or opinion considered particularly bad. The letter L is used as a slang term for loss (as in the opposite of win)."
In the insurance world, you sometimes see the phrase "L+ALAE Ratio" to refer to the ratio of (losses + expenses) divided by premium. It's a way to measure profitability for a book of insurance business: how many dollars of loss and expense do you have to pay per dollar of premium earned? Lower is better, and you don't want that ratio too much higher than 100%, because that means premiums aren't high enough to cover losses (though investment income can sustain small underwriting losses).
I could see "L+" used as shorthand for "L+ALAE" or "L+ALAE+ULAE," though admittedly, I've never seen that specific shorthand used.