Broadly, Capital Gains Tax (GCT) is the tax you pay on the sale of a capital asset. For example, shares in a company, or a property. The capital gain is generally the difference between what it cost you to acquire the asset and what you received when you disposed of it.
Where you are selling assets like shares or an investment property, you will likely have a capital gain/loss. If you are unsure whether you have capital gains or losses, you should seek further assistance from a tax advisor. You can find out more about CGT here.
Currently if you have made a capital gain or loss during the year you are not eligible to use Airtax for income tax purposes.