The flexibility of an Individual Savings Account (ISA) refers to whether you are able to withdraw money in a tax year from your ISA without affecting your annual allowance, which in the 2021/2022 tax year is £20,000.
If a cash ISA is flexible, it means you can withdraw money from it and put the money back into it within the same tax year without affecting how much you can invest in one year. Flexibility also applies to money you have invested in previous years.
In case you have a non-flexible ISA, the money you withdraw from your account reduces your remaining annual allowance that can be still used for that tax year. For example, if you had already reached the allowance limit and decided to take money out, you would not be able to put the money back in within that tax year.
Not all providers offer flexible ISAs: it is up to them if they offer this kind of product, and if so for which type of ISA. Before you sign up for an ISA, make sure you clarify the flexibility rules before signing a contract. Note that Lifetime ISAs (LISAs) and Junior ISAs (JISAs) cannot be flexible. Bear in mind also that even if your provider offers flexibility on fixed-rate ISAs, you will still need to pay a penalty fee for early withdrawal.