When something softens, it becomes less hard or rough. It can also mean to make someone feel less angry or upset.
Softening something is usually good because it makes things more comfortable or pleasant. Softening someone's feelings is also good because it helps people get along better.
Examples:
The butter softens in the sun.
She softens her voice when she talks to the baby.
He softens the ground before planting the seeds.
The music softens my heart.
Her smile softens his mood instantly.
The warm water softens the hard ice.