Updated: Apr 15, 2020
It means to increase the angle from the torso to the thigh.
However what is often seen is a 'fork seat', where riders inadvertently close the angle by tilting the pelvis back, often hollowing the lower back and wonder then why that area gets sore.
What this actually does is block the horses ability to come through from behind, round over his back, lower and engage the hind end.
This can often be caused through week core area in general, tight hip flexors (psoas muscles), glutes and lower back areas.
What you are looking to achieve (see below) takes time, your core is unlikely to be strong enough or developed yet unless you are either supplementing riding with other activity /sport that utilises the core, or you are riding multiple horses (professional rides generally) day in day out and have trained your core through that.
The pelvis must be tilted very slightly forward whilst pulling up through the upper body, the lower back must be almost flat, the back is used to support and stabilize the core – hence you must train it, it's not all about performing crunches..
Your hip flexors (psoas) muscles must be strengthened and stretched over time to allow the hip to become more open.