Current year earnings is an amount that sits on your balance sheet as the balancing figure between your assets, liabilities and equity. It is automatically calculated based on all of the expenses deducted from your income in the current year.
All of your income and all of your expenses.
What's not included
Any movements in your balance sheet (things that effect assets, liabilities or equity).
How is it taxed?
It is taxed at the corporation tax rate of 19% (currently as of 2019) when you come to do your tax return.