If FileVault is off, I know my disk is unencrypted but I don't quite understand what that means in regards to unauthorized access. How exactly would someone get into my Mac if it weren't encrypted, since I already have a password on it?
Every file is protected by permissions. Permissions are like laws: You can choose to follow them or not. Files are like property.
- John owns this file and controls who and what people can do
- Bob, Johns friend, can Read this file but can't Write to this file
- Sam, Johns colleague, can read and write to this file
- Everyone else is unable to read or write to this file
Note: You can see these permissions by selecting a file and pressing CMD + I. Under "Sharing & Permission" there is a list of users and there corresponding permissions.
It is up to the operating system/user to follow these laws (permissions).
Jack, Johns enemy, can look at the file and see that he does not have permission by John, the owner, to view this file. Jack is a bad guy and decides that he is going to look at the file anyway.
Jack can do this extremely easily by booting onto a flashdrive/external drive and following this guide: https://larryjordan.com/articles/mac-os-set-hard-drive-permissions/ or some many other various ways including booting from an external drive.
Scenario 1: The File is not Encrypted
Jack views the file and steals all the information on it.
Scenario 2: The File is Encrypted
Jack tries to view the file but the file is unreadable because it is encrypted. Jack yells "DARN!" in frustration and continues on to his next victim.
Scenario 3: The File is Encrypted but John Uses an easy to guess password Jack tries to view the file but the files is unreadable because it is encrypted. He decides to see if he can guess John's password and guesses "John123". Jack is correct and unencrypted the file. He yells "Stupid John" and steals all the information on it.
Moral of the story:
Permissions are like laws. Most law abiding operating systems and users will...well..abide by these laws. But some criminals (hackers or your average Joe) can very easily ignore these permissions. To prevent these people from accessing your information you have to encrypt the file with a strong password, otherwise the criminals can guess/brute force/hack the password and access your data.
FireVault encrypts your entire disk. I would suggest keeping it on if you have any important information on your disk.
The same way that anyone could get into any computer not knowing the username and password. Windows, Linux, macOS it's all the same from that perspective.
Im not being facetious here. If you have physical access to a computer there are lots of methods and hacker tools that will let you gain access to the computer.
And if the computer is taken it is a simple procedure to extract the HD/SSD and attach it to another computer, voila! complete access to the filesystem.
Really what most physical security is for is to keep casual users out of your system. A really knowledgable InfoSec pro with unlimited funds and access to high level hacking tools (governments, mostly...) can get into anything, it just depends on how valuable the info on your computer is to the presumed thief/hacker.
If you are a typical user with no state, or highly classified/illegal, info on your computer then your info is fairly safe behind your username and password. Only you can decide how much annoyance (EG passwords, encryption, etc.) you are willing to put up with to protect your data from slightly more than casual users and/or thieves that are likely to get their hands on your computer.