ISAs are tax-efficient wrappers. Every tax year, we each have an annual ISA allowance. If you don’t take full advantage of using all or part of it in one tax year, you cannot carry it over to the next.
There are various tax advantages to saving or investing through an ISA: you don’t pay Capital Gains Tax on any capital growth nor Income Tax on any income received, either as interest or dividends, from the investment or cash savings. Another advantage is that you don’t have to declare ISAs on your tax return.