See also: Notation#Stances
A stance is a unique state that grants use of new moves. In exchange it may limit access to blocking, movement or even portions of your normal movelist. Some stances also have evasive or autoparry properties. A stance might be entered freely, have optional entry only from certain moves, or mandatory entry from certain moves, depending on the character. Or a blend of all three properties.
For example, a character like Lili might take a risk to enter backturn stance and access a powerful 50/50. A character like Hwoarang continuously switches stance with most attacks to vary his pressure & confuse his opponent. Eddy, infamously, can evade some mids using his laying Negativa stance.