When you get a phone on a plan, the cost of the handset is subsided meaning you get a credit towards it's monthly cost.
For example: The latest phone costs $1200 brand new, this works out to be $50 a month. If you sign a $60 plan with Optus, they may give you $30 credit towards the handset. Meaning your total monthly cost is $80 ($60 for plan, $20 for phone). The handset credit/CR on your bill refers to the $30 subsidised credit.
If you didn't agree to the phone on a leasing plan, just a regular 24 month contract, then the phone is yours to keep.
If it's about that time, be sure to check out our SIM Only plans for some extra value if you don't plan on upgrading to a new phone.