A special defence of alibi means that the accused is claiming that, at the time the crime was committed, he/she was somewhere else (and therefore could not have done it). The word “alibi” comes from the Latin for “somewhere else”, so in that sense the clue is in the name.
An alibi has to specify the place and time (e.g. “between the hours of 8pm and 11pm, the accused was not at the locus, but was within his mother’s house at 123 Fake Street, Cumbernauld”).
As with any other special defence, it is for the Crown to lead evidence to disprove any claim of alibi.