From what I've seen many people do aerobics and weight training without involving their core to keep their balance. So what is core exercise ? Core exercises enable you to strengthen the core muscles including back,abs and pelvis. Having strong core make it easier to do physical activities, improve your posture, create body awareness, flexibility and flatten your stomach. Most importantly, avoiding lower back injury.
For me, core exercise is the toughest exercise ever. My personal trainer scheduled me to have different hour to train my core, since this type exercise you need to do it correctly. Took me 3 months of practice to be able do this exercise.
There are many core exercises, for beginners you may try bridge position. Beside train your core, this will help you to tone your glutes and harmstring.
a. Lie on the mat, put two fingers under your belly button and lock it. Open your feet as wide as your shoulder, your feet should facing to the front. Your palms facing up. Exhale and inhale through your nose. When you exhale feel your ribs widened and when you inhale feel your stomach getting smaller.
b. With same breathing, slowly lift your hips up and still locking your core. Do not arch your lower back.
c. When you are able to stabilize your body, lift one leg up ( without change the position ) and hold for 20 seconds then you switch with other leg.
Tips : Breathing is very essential. Breath slowly, you dont need to do many repetitions. If you can do it for 5 repetitions, you are fine.