A wallet is accessible by providing a unique, secret 24-word passphrase, or a secret 12-word passphrase. This is generated when creating a wallet, and should be stored in a secure location.
If you lose this passphrase, then access to the wallet will also be lost, and there is no way to recover the assets! Therefore, it's very important to safely store the wallet's Secret Recovery Phrase securely.
At the same time, anyone with the passphrase can access, and take all of the assets, so it's vital to make sure nobody else knows the passphrase. It's best practice to not have the passphrase saved on any computer.