Hi! I’m just a beginner yoga student. My teacher told me that my lower back is tight that is why I find it difficult to do the lotus pose or that posture where you bend forward to the ground in a normal cross-leg position. How do I make my lower back limber? I don’t have any back pain.
Do yoga. 
I added to my back problems by being too aggressive, so…
Work on a well-rounded practice. Do not aspire to achieve a pose, enter gradually, learn how your body works, keep it activated all the time. You are not a Yoga Journal cover.
Beware instruction from young people with athletic inclination or no history of injury.
Forward bends are the result of pelvic/hip/hamstring health, not spinal flexion. It’s ok not to be anywhere near the ideal pose. Explore props and modified poses on your way. Yoga is not about flexibility.
Smile!