It's both. 'Si' does mean 'if'. It can also mean 'Yes', but ONLY when you are contracting a previous comment. For example, we'd say it English:
So you don't like this?
Yes, I do!
and in French:
Alors, tu ne l'aimes pas?
Si, je l'aime.
The second person is using 'Si' to contradict the first person's assumption that he/she doesn't like whatever it is they're talking about. Otherwise, if there is no contradiction involved, you would just say 'Oui' for yes. This use of 'si' is common in Europe, but I believe it isn't used in Canada.