Explanation

Ossification means 'making new bone'. This happens gradually as cartilage changes into hard bone.

When a bone is still made of cartilage, and going through ossification, it is immature bone, because it is still growing. When a bone has no cartilage left and is made completely of hard bone, it is mature bone, because it has finished growing - ossification has happened. Some bones finish growing before you are a year old, but others keep growing until you are 25 years old!