To backup or export your wallet is pretty simple. This backup is the "key", usually a mnemonic (set of 24 words) or just a code that will give you access to your wallet if you lose your device, uninstall the app, etc. In short, if something wrong happens and you want/need to get access to your wallet again.
Things to know
On bluewallet every wallet has its own backup and there's no global backup for all wallets that you can create, this is something you need to do individually for each wallet. We advise to backup all wallets.
Where to do it
Inside your wallet you are going to find on the top right an options menu. Usually with the 3 dots icon. After you tap there you will be taken to the wallet options screen, where you will find the "backup/export" option.