Typically, an interview being requested/offered, means that the University thinks that (on paper), you appear intellectually capable of pursuing their Course to which you've applied.
Now they want to meet with you F2F, to check out your general attitude and that you're not a Nutbag... ;-)
Many Universities provide guidance on their website (possibly on the Subject pages, to which you've applied) - around what prep may be needed and/or what to expect at Interview - so, best track it down and read it.
A