This female is exhibiting lordosis.
Cat in lordosis, by Sidereal
During estrus, the queen will display behaviors to attract males; she will be extremely active and restless and will make loud calling sounds. She will also exhibit the lordosis posture. Lordosis, also sometimes called the coital crouch, is shown in this picture. The queen crouches low to the ground with her rear quarters elevated and her tail deflected to one side.
During estrus, the queen will not only allow mating to occur, but she will often actively seek out males.
This drive can be severe in many cases, and it's not uncommon to hear of indoor cats breaking out of their houses during estrus, in active pursuit of breeding males.