This method is used by Bitcoin Core (and few others) and comprises of backing up a wallet.dat file. It's a file that holds keys to spend outputs, addresses, transaction information, and other metadata such as contacts.
It has a different location based on an operating system.
Mac OS X: ~/Library/Application Support/Bitcoin/
For backup Locate wallet.dat and copy it to another device only you have an access to. It may be a USB drive, CD, another computer or your private home cloud storage.
You may also copy it to a cloud storage such as Mega or Dropbox but make sure it's encrypted.
To restore the backup, simply copy your backed up wallet.dat over an existing one in the bitcoin data location.