Capital Gains Tax is what you pay on the profit from selling an asset. So, if you bought a painting for £5,000 and sold it for £15,000, the taxable “gain” would be £10,000.
It is calculated as part of your Self Assessment Tax Return.
Capital Gains Tax is paid on things like property (other than your main home), shares, personal possessions worth over £6,000, and business assets.
You only pay Capital Gains Tax on gains that are over your tax-free allowance, which is £12,300 (or £6,150 for trusts).