Cardano (ADA) is natively a "proof of stake" (PoS) blockchain. With Cardano, you can earn rewards by delegating (or pledging) your ADA to a staking pool. Your delegated ADA can earn you rewards and help in the operation of the Cardano blockchain. Cardano staking is a way to earn rewards on the ADA you hold.
Staking your ADA does not involve selecting an amount to delegate - it stakes your entire Cardano wallet balance. You delegate your wallet address, not a portion of your ADA. This means you either stake all of the ADA in your wallet to a pool, or none of it. You can still send and receive ADA transactions as normal while your ADA is staked. When reward distribution occurs, your staked balance at that time is used to determine your rewards. You can still transact freely with your ADA while it is staked.
When you stake ADA, you are not giving up custody. You still hold your private keys and you control your ADA.
Arculus does not charge any fees for staking ADA. There is a 2 ADA deposit required to initiate staking. This is returned when you unstake. Additionally, some of the staking pools charge fees which are shown before you agree to stake your ADA.
When you choose to stake your ADA, you can choose from the publicly available list at https://cexplorer.io/pool. There are thousands of Cardano staking pools available. You must have at least a minimum of 4 ADA to stake.
There is no need to unstake your ADA in order to use it, but you always have the option to unstake. Before you unstake, however, you'll need to claim your rewards by tapping the "Rewards" button. Generally, though, you do not need to actively claim your rewards as your rewards are automatically included in your staked balance.
If you want to move to a different staking pool, you can do that whenever you want. The staking pool may charge a small fee. To redelegate, simply tap the "Redelegate" button and select the pool you want to delegate to. Remember, with Cardano you are delegating your address and your full quantity of ADA. It's not possible to delegate only part of your ADA holdings in your wallet. It is not necessary to claim your rewards to redelegate - the rewards are yours and stay with you even if you redelegate.