The harp is tuned in Equal Temperament, which is a universally accepted tuning system devised to make all semi-tones equal. The advantage of this system is that intervals between tones and semi-tones are the same in all keys. If the instrument were not in Equal Temperament, then you would need to tune each key individually. The big problem with tuning in an individual key is that you get one perfectly in tune key, but all other keys are noticeably out of tune – especially the octaves – and for modern ears, this makes for very uncomfortable listening.
The disadvantage with Equal Temperament tuning is that all intervals, except the octave are very fractionally out of tune. There is no need to be concerned about Equal Temperament: your electronic tuner will tune in this way automatically and it takes a highly trained ear to hear that the intervals are very slightly out of tune – but it is worth bearing in mind that in absolute terms, one feature of the harp is that it is never exactly in tune.