"navel center" refers to the area around your belly button or navel. It is often used in a metaphoric sense to describe a person's spiritual or emotional core. Full definition
This activates udiyana bandha, igniting the flames, and stimulates the energy around the navel center. (mindbodygreen.com)
Keep your shoulders soft and draw the navel center in towards the spine to engage your core and support your low back. (mindbodygreen.com)
Drop the tailbone between the hips (lifting the heels as needed) Squeeze the inner thighs onto the triceps as you shift your weight smoothly onto the hands, and firmly contract the abdominal muscles by pulling the navel center toward the spine, allowing the entire back to round (similar to cat pose) Keep the gaze forward as you glide onto the hands with the elbows bending to less than 90 degrees (crocodile arms). (yogafit.com)