Very often in class, you will hear the teacher say, "Start in B+," and immediately students will stand very straight in a croise position, with the back leg bent and the big toe on the floor, arms en bas (low 5th) and the knees and thighs touching.
B+ is a preparatory position that students take as they wait to go across the floor. Alternatively it is a pose dancers may use on stage - a very unassuming pose usually behind or around other dancers who may be performing. It's especially helpful when you're dancing on a stage which has no wings and you must stand patiently and unobtrusively as you are waiting your turn to dance!
The origins of the phrase, B+, are mixed. It's technically an "attitude a terre." Some say it comes from Labanotation which uses symbols for steps and that B+ is what the symbol for that step looks like. Others say it's from Balanchine and that the B stands for his name with the + indicating the back foot rests in a bent position while a - indicates the front foot is bent. I have never seen bent front foot as a preparation position although a straight leg tendu en efface as a preparation is often used particularly for big jumps. Many people think the RAD school (that's the Royal Academy of Dance) calls this step "classical pose."
In any case, this is what it looks like: