Your deep inner unit consists of four muscle groups that should work subconsciously to stabilize your pelvis, spine and ribcage under low load postures and movements like standing, bending and walking.  Accidents, injuries and developed muscle imbalances can cause portions of the deep inner unit to not do their job properly; the result can be pain and/or compensation from other muscle groups to try and brace to hold everything together.  Some of your other stronger muscles can make up for the deep inner unit, but this usually leads to too much compression on the joints and immobility in the area.  You function best when your body can use the little muscles to do light stuff and the bigger muscles to do harder stuff.  You can get away with purely building strength in your outer sling muscles, but you will be prone to breaking down more often if the little guys aren't firing.

The four muscle groups are your pelvic floor, transverse abdominus, multifidus and diaphragm.  They form the bottom, front, back and top of your abdominal and pelvic cavity.  Recruitment of these muscles is more about thinking than doing.  They provide gentle compression to stabilize so your bigger muscles can move you.  I don't like to re-invent the wheel so the best resource to learn about recruitment of these muscles can be found on Diane Lee's website here: Training the deep muscles of the core

‍Although becoming aware of these muscles and consciously training them can be very important, they are supposed to act subconsciously and if you align your body in the proper way they will likely fire on their own.  I find it is the compensation strategies people choose in their posture that are inhibiting these deep inner unit muscles and that helping a person unlearn bracing strategies helps to fire up the deep inner unit more than trying to focus on them alone.  Diane again touches on it with her article on butt grippers, back grippers and chest grippers.

You will find most of the Step 1 Videos on WTH are some form of teaching you to become aware of your gripping through exercise and movement.  Please read the two articles on Diane's website as a starting point and try to incorporate these muscles into the more advanced exercises you will find on WTH.