You’ll often hear the instruction “hug into the midline” or “use your core”. Here’s a couple of quick ways to help explain it.
You'll need a yoga block for the first exercise, if you don't have one try a folded cushion or blanket.
Finding your midline
Stand in Tadasana / Mountain Pose with your feet hip distance apart. Imagine a line running down the center of your body from your head and down the insides of your legs to your feet.
Place the block between your thighs and squeeze it towards that mid-line while also pushing it back by internally rotating your thighs. Keep your knees pointing forward though.
Avoid over-arching your back by keeping your lower belly in and length in your lower back
It doesn’t matter so much which way round the block is, it’s more about activating the inner leg muscles and educating your body about the actions of "drawing into the mid-line" and the internal rotation of the thighs.
Think about these actions in other poses. For example in wide-legged forward bend you can imagine trying to keep hold of a (very large!) block.
Think about your core before coming into the full pose.
Try this example:
Come into a low lunge with your right foot between your hands.
Drop the back heel and press down strongly through your feet.
Really activate your inner legs and lower belly by drawing your feet together without moving them. Remember that center line and actions of squeezing the block.
Pull the back of the belly in and up as you lift up into Warrior 2.
By drawing into your core and getting your alignment right before you come into the pose, vulnerable joints are protected. You are using your inner strength so that the outer body can stay soft.