Carryovers are generally created when you have some sort of tax benefit that cannot be fully used in the current year and it is carried over to a future year. Carryovers can be losses, deductions, or credits that are carried from one year to the future.
For example, some credits are nonrefundable, which means they only allowed to be used if you have enough income tax that taking the credit will not cause tax to be negative. They may allow you to carry the unused portion to a future year, when you have more tax.