While not part of my official Opera Stage Management 101 series, I'm tagging this one as such, too. When you're reading music, there can often be a new symbol that pops up that may be unfamiliar. It appears that composer Benjamin Britten created one of his own, now called a curlew sign. Here's a post from my friend soprano Emily Pulley, as posted on Facebook today.
(Here's the link to the full post for bigger images.)
According to the last image she posted, a curlew sign is something he invented for when there is no conductor, first for his production of Curlew River. It indicates that the singer or instrumentalist must listen and wait until the other performers have reached the next bar line or meeting point, a bit like a fermata or pause compared to the written music.
Or you can call it "upside down boobs"...