These exercises are good for everyone, regardless of whether you have back pain, but in particular they are helpful for people with lower back pain, which can have several causes such as sciatica, spondylolisthesis, herniated discs, weak or tight muscles and so on.
This link has a selection of back exercises which apply to everyone, although designed to assist people with general lower back pain.
This link has a selection of back exercises which apply to everyone, although designed to assist people with spondylolisthesis.
This link has a selection of back exercises which apply to everyone, although designed to assist people with sciatica.
You can google 'exercises for lower back pain' and find many more exercises for easing lower back pain.
Here are some exercises for hyperkyphosis and
explanation and exercises for Scheuerman's Disease.