Tight Hip Flexors from Anatomical StandPoint

Hello.
What exactly means tight hip flexors from anatomical standpoint?
If tight hip flexors means PSOAS and PSOAS is attached to the spisnal erectors and to the quads, Am I need to lengthing/longer the PSOAS.
What exactly means to lenghting the PSOAS - to push my legs and glutes forward and downward,and my lower back - backward and downward.

From my experience when I am straight and push my legs forward and downward, It prevents spacula from anteior tilt.
But In this possition my ABS still stick out and my lower back is arched.
Am I need to push my upper legs and glutes forward, and in the same time to brace my ABS and keep my lower back backward and downward.

It is important for me, because I want to understand the things from functional anatomical standpoint.

I can’t understand how to manipulate my legs and my lower ABS for optimal PSOAS (hip flexors) position during the exercises? Is the rigion in the circle on the second picture must be straight and forward?
Thank you.