Strengthening your back helps to prevent back pain by supporting your lower back and improving your overall stability. There are certain muscles that you should be training in order to strengthen your back such as:

• Lower back muscles: Your lower back muscles provide stability and support to your spine. Exercises such as the superman and back extensions can help to strengthen your lower back muscles.
• Trapezius: The trapezius muscle is located in your upper back to neck area. It helps to maintain good shoulder movement and posture. Exercises like shoulder presses can help to strengthen your trapezius. 
• Erector spinae: Erector spinae is a group of muscles that helps to maintain proper posture and extension in your spine. Exercises like deadlifts and hyperextensions can help to strengthen your erector spinae.