Before 5 to 6 months of age a baby will usually begin to roll from it's belly to it's back.
They usually use their arms to allow them to move from their belly to their back and then back again.
By 5 to 6 months of age your baby will then be able to move from it's back to it's belly which is the opposite of the baby moving from it's tummy to it's back.
To move from their back to their tummy they'll need stronger muscles.
When doing Tummy time that's when they usually start rolling over.