Carry-forward lets you use concessional cap you didn't use in earlier years. If you contributed less than the cap, the unused amount rolls forward for up to 5 years.
The $500,000 balance test
You can only use carry-forward amounts if your total super balance was under $500,000 on 30 June of the prior financial year. Above that, only the standard $30,000 cap applies.
Worked example
If you used only $10,000 of your cap in each of the last three years, you've banked roughly $60,000 of unused cap (3 × ~$20,000–$27,500 depending on the year's cap). In a high-income year you could make a large deductible contribution to soak up that space and cut your tax bill — provided your balance is under $500k.