When thinking about bio mechanics (the study of the mechanical laws relating to the movement or structure of living organisms) the core works best to help stabilize and support the pelvis and lumbar spine when in a neutral position.
When standing or sitting with a neutral pelvis, the action of gravity on the trunk musculature leads to balanced engagement of the muscles around the spine and abdomen.
This is very important to understand because this is what decreases the stress on the spine and helps to prevent low back pain and injury.
The body has different landmarks that can be used to identify a neutral pelvis. The easiest way to identify neutral is when the anterior superior iliac spines (ASIS) which are the bony protrusions in the inside of the hips are leveled with the public bone.
Place the heel of your palms on the ASIS and with the second or third fingers, bring them together and point them towards the public bone. You are creating a diamond shape. Notice if the diamond is leveled or tipped.
If the top of your diamond is anterior to the pubic bone, that is what's called an Anterior pelvic tilt. If the top of your diamond is posterior to the pubic bone, that means that the pelvis is posteriorly tilted.
Rock the pelvis back and forth until the ASIS is leveled with the pubic bone. Once leveled, that is what is considered a neutral pelvis.
In terms of imagery queing, imagine you are lying down, you have your diamond shape leveled, from there, imagine a cup of hot coffee in the middle of your leveled diamond. Don't spill it! Maintain that position of neutral when moving to avoid getting burned.
This neutral pelvis is not a fixed position but rather a dynamic concept that shifts and changes slightly in relationship with movement being performed.