A standalone account is an Algorand address and private key pair that is not stored on disk. The private key is most often in the 25-word mnemonic form.
Standalone accounts have a low setup cost as you do not need to connect to a separate client that depends on separate hardware. All you need is the 25-word human-readable mnemonic of the relevant account.
You can later import the created account to any wallet supporting Algorand using the mnemonic phrase.