How do UNIQUE indexes work on multiple columns?

just a quick question: how exactly does the unique index check the if the field(s) is(are) unique when added on multiple columns?

For example, if I add a UNIQUE index on the email,password columns, will the index check if:

1. Is email unique AND is password unique, or


2. The combination of email,password is unique, as in, it would allow u1@mail.com, 123 and u2@mail.com, 123

Apologies if my thoughts seem abrupt here, I'm having difficulty in expressing them today for some reason. Thanks in advance!

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