What Are Loose Accounts?
Suppose you used the Import Account capability to import an account. Now, when you click Switch Accounts, you see the word "Loose" with a bright red oval background superimposed on your account avatar.
The "Loose" label means that the account is not backed up by your seed phrase. This happened because you imported the account after creating your seed phrase.
As a precaution, keep the information you were using to access this "Loose" account backed up outside of MetaMask. (Similar to how you keep your seed phrase backed up somewhere offline.) This way, if something goes wrong, you can recover the account.