Firstly, Bracing...Inhale a big diaphragmatic breath (fill your stomach/abdomen with air filling it up like a balloon), and then draw your belly button in. This is what I refer to when saying activate your core or keeping core tight. This activates the inner unit and will provide stability and integrity to the lower back/spine, and also helps with your power output. Like any skill this takes practice.
Second "Core Strength" - this isn't doing sit ups and working your abdominals
Core exercises train the muscles in your pelvis, lower back, hips and abdomen to work in harmony with each other.
Your core muscles should ideally be working every minute of every day and every rep you do in your resistance sessions.
Your core muscles are the muscles deep within the abdominals and back, attaching to the spine or pelvis - transverse abdominis, multifidus, internal and external obliques, erector spinae, diaphragm, pelvic floor muscles.
The 'Dead Bug' is a great 'core' exercise for, used to target the trunk muscles (erector spinae, obliques, rectus abdominis, transverse abdominis) which together provide a strong base for our moving limbs so we can maintain good form whilst running/walking.
Engaging your core means contracting your trunk muscles above to provide support for your trunk/core in static (still) positions and during dynamic (moving) movements. These muscles are used for balance, lifting, pushing, pulling, and general movement.
A strong core helps improve balance, decrease injuries, and support your spine during intense movements.
Overall, your core muscles are involved in the stability and mobility of the spine. They’re the “core” of all movements that your body does throughout the day and your workouts and are not 'your visible abs.'