Capital gains tax is a tax on the profit when you sell or dispose of something that has risen in value. It is the gain that is taxed, not the total amount you receive.
It can apply to things like a second property, investments held outside a pension or ISA, and some personal possessions. Everyone has an annual allowance, and the gain above it is what may be taxed. How much depends on your wider income and circumstances, which is where planning earns its keep.

