An online wallet is a digital wallet in which you can save and keep your bitcoins securely. A unique public address is linked to each wallet. This address can be compared to a bank account number.
You can create a Bitcoin wallet in a variety of ways. You can find an overview at https://bitcoin.org/en/choose-your-wallet
of different Bitcoin wallets.
An easy way of creating an online wallet is via https://www.blockchain.info
. Here you can create a wallet in only a few minutes that you can use everywhere.