Planks are great to work your core. This strengthens your spine, rhomboids, trapezius and ab muscles which naturally builds a strong posture and balance, tones and also reduces back pain as well.
Tip: Keep your head and spine neutral as you engage your core lifting your hips and knees off the mat (or floor). Do not let your stomach drop and your hips to rise up. To modify, place your knees on the mat for an easy option.
20 seconds x planks
10 seconds x active recovery
4 sets (or rounds)
Do these with a mat or without one. You can repeat these as much as you can to challenge yourself. Good luck!